Update README for the new 3.3 branch

Author: aaronfranke

README.md

@@ -6,9 +6,11 @@ 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 stable Godot version (currently 3.3.x).
+- The [`master`](https://github.com/godotengine/godot-demo-projects) branch is compatible with the latest stable Godot version (currently 3.4.x).
 - If you are using an older version of Godot, use the appropriate branch for your Godot version:
 
+  - [`3.3`](https://github.com/godotengine/godot-demo-projects/tree/3.3)
+    branch for Godot 3.3.x.
   - [`3.2`](https://github.com/godotengine/godot-demo-projects/tree/3.2)
     branch for Godot 3.2.x.
   - [`3.1`](https://github.com/godotengine/godot-demo-projects/tree/3.1)