Update the PR template and Contributing Guidelines (#738)

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,8 +2,10 @@
 
 Please include the PEP number in the pull request title, example:
 
-```
 PEP NNN: Summary of the changes made
-```
 
--->
+In addition, please sign the CLA.
+
+For more information, please read our Contributing Guidelines (CONTRIBUTING.rst)
+
+-->

CONTRIBUTING.rst

@@ -8,9 +8,16 @@ PEP 0, the index PEP, is now automatically generated, and not committed to the r
 Before writing a new PEP
 ------------------------
 
-Start with a discussion in `python-ideas <https://mail.python.org/mailman/listinfo/python-ideas>`_
-mailing list to determine if the idea is PEP-able. More details about it in
-`PEP 1 <https://www.python.org/dev/peps/pep-0001/#start-with-an-idea-for-python>`_.
+Has this idea been proposed on `python-ideas <https://mail.python.org/mailman/listinfo/python-ideas>`_
+and received general acceptance as being an idea worth pursuing? (if not then
+please start a discussion there before submitting a pull request).
+
+More details about it in `PEP 1 <https://www.python.org/dev/peps/pep-0001/#start-with-an-idea-for-python>`_.
+
+Do you have an implementation of your idea? (this is important for when you
+propose this PEP to `python-dev <https://mail.python.org/mailman/listinfo/python-dev>`_
+as code maintenance is a critical aspect of all PEP proposals prior to a
+final decision; in special circumstances an implementation can be deferred)
 
 
 Commit messages
@@ -20,6 +27,23 @@ When committing to a PEP, please always include the PEP number in the subject
 title. For example, ``PEP NNN: <summary of changes>``.
 
 
+Sign the CLA
+------------
+
+Before you hit "Create pull request", please take a moment to ensure that this
+project can legally accept your contribution by verifying you have signed the
+PSF Contributor Agreement:
+
+    https://www.python.org/psf/contrib/contrib-form/
+
+If you haven't signed the CLA before, please follow the steps outlined in the
+CPython devguide to do so:
+
+    https://devguide.python.org/pullrequest/#licensing
+
+Thanks again to your contribution and we look forward to looking at it!
+
+
 Code of Conduct
 ---------------