Recommend Self in CONTRIBUTING.md (#5699)

Part of #5676.

Co-authored-by: Akuli <akuviljanen17@gmail.com>

CONTRIBUTING.md

@@ -373,6 +373,10 @@ are not sure whether exceptions are suppressed or not or if the
 context manager is meant to be subclassed, pick `Optional[bool]`.
 See https://github.com/python/mypy/issues/7214 for more details.
 
+`__enter__` methods and other methods that return instances of the
+current class should be annotated with the `_typeshed.Self` type
+variable ([example](https://github.com/python/typeshed/pull/5698)).
+
 A few guidelines for protocol names below. In cases that don't fall
 into any of those categories, use your best judgement.