Add a README and screenshots to the OS test demo

misc/os_test/README.md

@@ -0,0 +1,37 @@
+# Operating System Testing
+
+This demo showcases various OS-specific features in Godot.
+It can be used to test Godot while porting it to a
+new platform or to check for regressions.
+
+In a nutshell, this demo shows how you can get information from the
+operating system, or interact with the operating system.
+
+Language: GDScript and some [C#](https://docs.godotengine.org/en/latest/getting_started/scripting/c_sharp/index.html)
+(Mono is NOT required to run this demo)
+
+Renderer: GLES 2
+
+## How does it work?
+
+The [`OS`](https://docs.godotengine.org/en/latest/classes/class_os.html)
+class provides an abstraction layer over the platform-dependent code.
+OS wraps the most common functionality to communicate with the host
+operating system, such as the clipboard, video driver, date and time,
+timers, environment variables, execution of binaries, command line, etc.
+
+The buttons are connected to a node with the `actions.gd` script, which
+perform actions using the OS class.
+The text on the left is filled in using the `os_test.gd` script,
+which gathers information about the OS using the OS class.
+
+On a Mono-enabled version of Godot, Godot will load `MonoTest.cs` into
+the `MonoTest` node. Then, information determined by
+[`C# preprocessor defines`](https://docs.godotengine.org/en/latest/getting_started/scripting/c_sharp/c_sharp_features.html#preprocessor-defines)
+will be added to the left panel.
+
+## Screenshots
+
+![Top HiDPI](screenshots/top-hidpi.png)
+
+![Mono](screenshots/mono.png)

misc/os_test/actions.gd

@@ -1,4 +1,4 @@
-extends VBoxContainer
+extends Node
 
 
 func _on_OpenShellWeb_pressed():

misc/os_test/os_test.gd

@@ -1,4 +1,4 @@
-extends Panel
+extends Node
 
 onready var rtl = $HBoxContainer/Features
 onready var mono_test = $MonoTest