Add demo submission guidelines (#912)

The pull request template was changed to match godotengine/godot's.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,23 +1,7 @@
 <!--
-Only submit a pull request if all of the following conditions are met:
+Please target the `master` branch in priority.
+PRs can target `3.x` if the same change was done in `master`, or is not relevant there.
 
-* It must work with the latest Godot version of the branch you're submitting to.
-
-* It must follow all of the Godot style guides, including the GDScript
-  style guide and the C# style guide.
-
-* The demo should not be overcomplicated. Simplicity is usually preferred.
-
-* If you are submitting a new demo, please ensure that it includes a
-  README file similar to the other demos.
-
-* If you are submitting a copy of a demo translated to C# etc:
-
-    * Please ensure that there is a good reason to have this demo translated.
-      We don't want to have multiple copies of every single project.
-
-    * Please ensure that the code mirrors the original closely.
-
-    * In the project.godot file and in the README, include "with C#" etc in
-      the title, and also include a link to the original in the README.
+Relevant fixes are cherry-picked for stable branches as needed by maintainers.
+You can mention in the description if the change is compatible with `3.x`.
 -->

CONTRIBUTING.md

@@ -0,0 +1,109 @@
+# Contributing to Godot demo projects
+
+Thanks for your interest in contributing to the Godot demo projects!
+
+## Demo submission criteria
+
+Please follow these guidelines for submitting new demos or improving existing demos:
+
+- The demo must work with the latest Godot version of the branch you're submitting to.
+
+- The demo must follow all of the Godot style guides:
+  - [Project organization](https://docs.godotengine.org/en/stable/tutorials/best_practices/project_organization.html)
+  - [GDScript style guide](https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_styleguide.html)
+    - In GDScript, type hints should be used to improve runtime performance
+    and ease code maintenance. Do not use type inference syntax
+    (`var some_integer := 123`) if the resulting type is not obvious from
+    reading the righthand side of the assignment.
+  - [C# style guide](https://docs.godotengine.org/en/stable/tutorials/scripting/c_sharp/c_sharp_style_guide.html)
+  - [Shaders style guide](https://docs.godotengine.org/en/stable/tutorials/shaders/shaders_style_guide.html)
+
+- The demo should not be overcomplicated. Simplicity is usually preferred.
+
+- The submitted files must be freely redistributable and modifiable,
+  including for commercial use. This applies to art assets as well.
+  - In practice, this means the following licenses are accepted for art assets:
+    - CC0 1.0 (or similar public domain dedications)
+    - CC BY 3.0/4.0
+    - CC BY-SA 3.0/4.0
+  - Licenses such as "Royalty-free" (without clarification), CC BY-NC or
+    CC BY-ND are **not** accepted.
+
+- To allow demos to be cloned quickly, file sizes should be kept reasonable for
+  all files submitted. Try to keep the demo project files's download size below
+  20 MB, unless there is a good reason to have a larger demo (e.g. if
+  high-quality textures are *required* to show off an effect).
+  - You can check this by creating a ZIP file containing the project files,
+    then removing `.godot/` within the ZIP file.
+  - For 3D demos, consider relying on Godot's procedural generation abilities
+    (such as [NoiseTexture2D](https://docs.godotengine.org/en/stable/classes/class_noisetexture2d.html))
+    to decrease file size.
+
+### Submission criteria for new demos
+
+If you are submitting a new demo:
+
+- Make sure it includes a `READE.md` file similar to the other demos.
+
+- Make sure a short description (1-3 lines) is set in the Project Settings.
+
+- Make sure the project has a `icon.webp` in lossless WebP[^1] format that is
+  128×128 pixels, with its path defined in the Project Settings. This allows the
+  icon to remain crisp on hiDPI displays.
+
+- Make sure the project includes a `screenshots/` folder containing a screenshot in
+  lossless WebP[^1] format similar to the other demos. It's recommended to stick to
+  only 1 screenshot, but you may add multiple screenshots if required. The
+  screenshot should be in 1280×720 resolution, without visible window borders.
+  - The `screenshots/` folder must contain an empty `.gdignore` file to prevent
+    Godot from importing screenshots as resources.
+
+- Make sure the demo has keyboard and controller input support (plus mouse if
+  relevant). Explicit touch input support is a bonus, but not required.
+  - For keyboard events, WASD configurations must use *physical* key locations
+    in the Input Map to allow them to work out of the box on non-QWERTY keyboard
+    layouts.
+
+#### Visual considerations
+
+- Unless the project requires a different stretch mode and aspect, use the
+  `canvas_items` stretch mode and `expand` stretch aspect in the Project
+  Settings. Configure anchors for Control nodes correctly, so that
+  [multiple resolutions](https://docs.godotengine.org/en/stable/tutorials/rendering/multiple_resolutions.html)
+  and aspect ratios are supported.
+
+- Demos that don't update their visual output often (such as UI demos) must have
+  **Low Processor Mode** enabled in the Project Settings. This must be left
+  disabled in demos where constant redrawing is expected (such as most "game"
+  demos).
+
+- For greater performance and compatibility, use the **Compatibility** rendering
+  method in 2D demos that don't require advanced graphics functionality.
+  - In 3D demos or 2D demos that require advanced graphics functionality, use
+    the **Forward+** rendering method (**Mobile** is automatically used on
+    mobile platforms in this case).
+
+- In 3D projects, set **Rendering > Textures > Default Filters > Anisotropic Filtering Level**
+   to **16×** in the Project Settings. Make sure all materials use the relevant
+   anisotropic sampler mode (**Nearest Mipmaps Anisotropic** or **Linear Mipmaps Anisotropic**).
+- In 3D projects, set **Rendering > Anti Aliasing > Quality > MSAA 3D** to **4×**.
+  - In projects that use demanding graphics features such as SDFGI, set
+  **Rendering > Anti Aliasing > Quality > Screen Space AA** to **FXAA** instead
+  as this is faster.
+- Use graphics options that perform reasonably well on low-end to mid-range
+  hardware. Demanding graphics options should not be enabled by default, unless
+  they're the entire point of the demo.
+
+#### If you are submitting a copy of a demo translated to C#
+
+- Please ensure that there is a good reason to have this demo translated. We
+  don't want to have multiple copies of every single project, as this makes
+  maintenance more difficult.
+
+- Please ensure that the code mirrors the original as closely as possible.
+
+- In the `project.godot` file and in the `README.md`, include "with C#" in
+  the title, and also include a link to the original in the `README.md`.
+
+[^1]: You can use [Squoosh](https://squoosh.app/) to convert images to WebP.
+Make sure to enable the Lossless option, as it's not the default.