Refactor the plugin demos to be inside of a project for convenience

plugins/README.md

@@ -1,9 +1,35 @@
-# Plugin demos
+# Plugin Demos
 
-To use these plugins, copy any of these folders to the `addons/` folder in a Godot project.
+This contains multiple plugin demos, all placed in a project for convenience.
+
+Please see the documentation for editor plugins:
+https://docs.godotengine.org/en/latest/tutorials/plugins/editor/index.html
+
+Language: GDScript
+
+Renderer: GLES 2
+
+# How does it work?
+
+This project contains 3 plugins:
+
+* The custom node plugin shows how to create a custom node type
+  using `add_custom_type`. [More info](addons/custom_node).
+
+* The material import plugin shows how to make a plugin handle importing
+  a custom file type (mtxt). [More info](addons/material_import_plugin).
+
+* The material creator plugin shows how to add a custom dock with some
+  simple functionality. [More info](addons/material_creator).
+
+To use these plugins in another project, copy any of these
+folders to the `addons/` folder in a Godot project, and then
+enable them in the project settings menu.
 
 For example, the path would look like: `addons/custom_node`
 
-Plugins can be distributed and installed from the UI. If you make a zip that contains the folder, Godot will recognize it as a plugin and will allow you to install it.
+Plugins can be distributed and installed from the UI.
+If you make a zip that contains the folder, Godot will recognize
+it as a plugin and will allow you to install it.
 
 This can be done via the terminal: `zip -r custom_node.zip custom_node/*`