Add comment about Mypy plugins

Author: hynek

changelog.d/887.breaking.rst

@@ -12,3 +12,5 @@ The API docs have been mostly updated, but it will be an ongoing effort to chang
 Please note that we have **not** moved -- or even removed -- anything from ``attr``!
 
 Please do report any bugs or documentation inconsistencies!
+
+Please note that until Mypy adds support for the new namespace, you'll have to explicitly tell it about it as documented `here <https://www.attrs.org/en/stable/extending.html#mypy>`_.