Add CONTRIBUTING.rst

Add contributing guidelines taken almost verbatim from the commando
project (removes the requirement to update a version because new
releases may include more than one set of changes). Fixes #303.

CONTRIBUTING.rst

@@ -0,0 +1,45 @@
+**tl;dr**  Good (code + tests + commit message) = Great Pull Request.
+
+*********************************************************************
+
+How do the pull requests get merged?
+------------------------------------
+
+The following points are considered as part of merging pull requests
+after it is deemed necessary.
+
+1.  Is there an issue tagged in the commit?
+2.  Do the existing tests pass?
+3.  Are there new tests added to verify any new functionality / issue?
+4.  Is the authors list up to date?
+5.  Is the changelog updated?
+6.  Is the version updated?
+7.  Does this require any changes to the documentation?
+
+Guidelines
+----------
+
+If the following guidelines are observed as much as possible, it will
+immensely help in verifying and merging the pull requests.
+
+1.  One pull request = One feature or One bug.
+2.  Always tag an issue in the commit. If an issue does not exist for
+    a feature or a bug, please add one.
+3.  Use topic / feature branches.
+4.  Make sure a test exists to verify the committed code. A good way
+    to think about it is: if these commits were reversed and only the
+    test were added back in, it ought to fail.
+5.  Make the `commit message`_ as verbose as possible.
+6.  Add yourself to `Authors`_ list and update your contribution.
+7.  Cross update `Changelog`_ list as well.
+8.  If the change was complicated and resulted in a lot of commits,
+    consider ``rebase -i`` to squash and/or rearrange them to make it
+    easier to review.
+9. Update the `Readme`_.
+
+
+.. _commit message: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+.. _Changelog: CHANGELOG.rst
+.. _Authors: AUTHORS.rst
+.. _Readme: README.rst
+