Document the requirement to update the class reference when contributing
Document the requirement to update the class reference when contributing

Documenting new additions has been an informal requirement for
a few months now. This makes it official.

(cherry picked from commit 6d0e8f9fb70ff00088185f59a627c8f65f5d78ed)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot