Add allow-branch hook update instructions.

pep-0101.txt

@@ -38,6 +38,9 @@ Things You'll Need
     * Access to ``dinsdale.python.org``, the Python web host.  You'll
       be uploading files directly here.
 
+    * Shell access to ``hg.python.org``, the Python Mercurial host.  You'll
+      have to adapt repository configuration there.
+
     * Write access to the website SVN repository
 
       Instructions here:
@@ -266,6 +269,12 @@ How to Make A Release
           $ hg update deadbeef    # revision ID noted down before
           $ hg branch 3.2
 
+      ___ When you want to push back your new branch to the main CPython
+          repository, the new branch name must be added to the "allow-branches"
+          hook configuration, which protects against stray named branches being
+          pushed.  Login to hg.python.org and edit (as the "hg" user)
+          ``/data/hg/repos/cpython/.hg/hgrc`` to that effect.
+
   ___ Push your commits to the remote release clone.
 
       $ hg push ssh://hg.python.org/releasing/...