Note about changelog entries for changes not in cpython.

Author: cjw296

docs/changelog.txt

@@ -1,4 +1,4 @@
-Changelog from Python's News
-============================
+Changelog
+=========
 
 .. include:: ../CHANGELOG.rst

docs/index.txt

@@ -59,8 +59,8 @@ with Mock functionality should be reported to the `Python bug tracker
 
 .. index:: python changes
 
-Python Changes
-++++++++++++++
+Changelog
++++++++++
 
 See the :doc:`change log <changelog>`.
 
@@ -78,7 +78,11 @@ Committers can just push as desired: since all semantic development takes
 place in cPython, the backport process is as lightweight as we can make it.
 
 mock is CI tested using Travis-CI on Python versions 2.7, 3.4,
-3.5, 3.6, nightly Python 3 builds, pypy, pypy3.
+3.5, 3.6, pypy, pypy3.
+
+If you end up fixing anything backport-specific, please add an entry
+to the top of ``CHANGELOG.rst`` so it shows up in the next release
+notes.
 
 Releasing
 ---------