Add a link to the code of conduct in the repository

This makes it display when opening an issue for the first time in the repository.

CODE_OF_CONDUCT.md

@@ -0,0 +1,4 @@
+# Code of Conduct
+
+By participating in this repository, you agree to abide by the
+[Godot Engine Code of Conduct](https://godotengine.org/code-of-conduct).