Linked to file codec documentation

Author: radarhere

docs/releasenotes/9.1.0.rst

@@ -157,7 +157,8 @@ Added PyEncoder
 ^^^^^^^^^^^^^^^
 
 :py:class:`~PIL.ImageFile.PyEncoder` has been added, allowing for file encoders to be
-written in Python.
+written in Python. See :ref:`Writing Your Own File Codec in Python<file-codecs-py>` for
+more information.
 
 Other Changes
 =============

src/PIL/ImageFile.py

@@ -702,6 +702,8 @@ class PyEncoder(PyCodec):
     """
     Python implementation of a format encoder. Override this class and
     add the decoding logic in the :meth:`encode` method.
+
+    See :ref:`Writing Your Own File Codec in Python<file-codecs-py>`
     """
 
     _pushes_fd = False