Update pep 12 to mention rst extension.

Author: Carreau

pep-0012.txt

@@ -24,7 +24,7 @@ the text (reStructuredText) source of this PEP in order to complete
 the steps below.  **DO NOT USE THE HTML FILE AS YOUR TEMPLATE!**
 
 The source for this (or any) PEP can be found in the PEPs repository,
-viewable on the web at https://hg.python.org/peps/file/tip .
+viewable on the web at https://github.com/python/peps/ .
 
 
 Rationale
@@ -53,11 +53,13 @@ or contact the PEP editors <[email protected]>.
 Once you've decided which type of PEP yours is going to be, follow the
 directions below.
 
-- Make a copy of this file (``.txt`` file, **not** HTML!) and perform
-  the following edits.
+- Make a copy of this file (``.txt`` or ``.rst`` file, **not** HTML!)
+  and perform the following edits. Prefer ``.rst`` extensions as this
+  will allow PEPs to be rendered on GitHub. ``.txt`` extensions still
+  exists for legacy reasons.
 
-- Replace the "PEP: 12" header with "PEP: XXX" since you don't yet have
-  a PEP number assignment.
+- Replace the "PEP: 12" header with "PEP: XXX" since you don't yet
+  have a PEP number assignment.
 
 - Change the Title header to the title of your PEP.