Mention the "formate-black" extension.

domdfcoding · domdfcoding · commit 61b30d46990e · 2021-06-06T20:03:41.000+01:00
diff --git a/doc-source/extending.rst b/doc-source/extending.rst
@@ -107,3 +107,8 @@ They can request this using the :deco:`formate.config.wants_filename` decorator
 		...
 
 		return reformatted_source
+
+
+-----
+
+See :github:repo:`repo-helper/formate-black` for an example extension.
