Clarify status of this repo with respect to Godot 4

It's unclear to me what the status of this repository is with respect to Godot 4 and the new GDExtension API. This readme and `godot-cpp`'s readme have similar Versioning notes at the top, saying that `master` is being kept in line with Godot's `master` and that branches exist for `3.x` stable releases. However, whereas `godot-cpp` has clearly been updated for GDExtension, as far as I can tell this repo has no GDExtension support, even on `master`. The header is still called `gdnative_interface.h`, and it's missing several newer interface functions, which causes the whole header to be out of sync with Godot master.

If this repo is only intended for use with Godot 3 and GDNative, then that should be made clear in the readme. If this repo is going to be updated to GDExtension and has not yet, then that should also be made clear (and a link to the place in the Godot repo where `gdextension_interface.h` can be found in the meantime might be useful as well).