add compatibility warning

README.md

@@ -2,6 +2,10 @@
 
 GDExtension template that automatically builds into a self-contained addon for the Godot Asset Library.
 
+# Compatibility warning:
+
+Godot 4.0 is still in active development, so there will be frequent incompatibilities with previous versions. If you are encountering an issue, make sure the submodule `godot-cpp` is up-to-date by running the command `git submodule update --remote`.
+
 ### Getting started:
 1. Clone this repository (or a new repository with this template) with submodules.
     - `git clone --recurse-submodules https://github.com/nathanfranke/gdextension.git` \