Clarify that master here tracks 3.x upstream (#764)

Author: akien-mga

README.md

@@ -6,7 +6,8 @@ be used with [Godot Engine](https://godotengine.org), the open source
 
 ## Godot versions
 
-- The [`master`](https://github.com/godotengine/godot-demo-projects) branch is compatible with the latest Godot version (currently 4.0 alpha).
+- The [`master`](https://github.com/godotengine/godot-demo-projects) branch is compatible with Godot's `3.x` branch.
+- The [`4.0-dev`](https://github.com/godotengine/godot-demo-projects/tree/4.0-dev) branch is compatible with Godot's `master` branch (currently 4.0-alpha).
 - If you are using an older version of Godot, use the appropriate branch for your Godot version:
 
   - [`3.4`](https://github.com/godotengine/godot-demo-projects/tree/3.4)