Reorganize README and add a notice about engine version compatibility

README.md

@@ -4,12 +4,21 @@ Each folder containing a `project.godot` file is a demo project meant to
 be used with [Godot Engine](https://godotengine.org), the open source
 2D and 3D game engine.
 
-Those demos are distributed under the terms of the MIT license, as
-described in the [LICENSE.md](LICENSE.md) file.
+## Important note
 
-### Useful links
+- The `master` branch is compatible with the latest Godot version (currently 3.0).
+- Use the appropriate branch for your Godot version, such as the
+  [`2.1`](https://github.com/godotengine/godot-demo-projects/tree/2.1) branch
+  for Godot 2.1.x.
+
+## Useful links
 
  - [Main website](https://godotengine.org)
  - [Source code](https://github.com/godotengine/godot)
  - [Documentation](http://docs.godotengine.org)
  - [Community hub](https://godotengine.org/community)
+
+## License
+
+Those demos are distributed under the terms of the MIT license, as
+described in the [LICENSE.md](LICENSE.md) file.