Docs: Add CONTRIBUTING guide

Fixes #3999.

CONTRIBUTING.rst

@@ -0,0 +1,68 @@
+How to contribute
+=================
+
+Reporting bugs
+--------------
+
+Before reporting please:
+
+- Double-check `the documentation
+  <http://docs.translatehouse.org/projects/pootle/>`_.
+- Make sure `the issue hasn't been already reported
+  <https://github.com/translate/pootle/issues/>`_.
+
+
+Write your bug report so that you ensure developers are fixing the bug rather
+than spending time trying to understand it.  If we can't see it or replicate it,
+likely we can't fix it.
+
+Checklist:
+
+#. Bug description is clear yet concise.
+#. Clear instructions to reliably replicate the issue.
+#. Screenshots rather than descriptions of screens.
+#. Full traceback if one occurred.
+#. Links to the actual issue.
+
+
+Commits and Pull Requests
+-------------------------
+
+Before contributing any code, please:
+
+- Do not write any code without consulting about it first, unless it is a
+  trivial fix. Someone else can be already working on it.
+- Think if any code is needed at all: `The Best Code is No Code At All
+  <http://blog.codinghorror.com/the-best-code-is-no-code-at-all/>`_.
+
+
+Keep the commit log as healthy as the code. It is one of the first places new
+contributors will look at the project.
+
+#. No more than one change per commit. There should be no changes in a commit
+   which are unrelated to its message.
+#. Every Pull Request should pass all tests on its own. Ideally each commit
+   should too.
+#. Follow `these conventions <http://chris.beams.io/posts/git-commit/>`_ when
+   writing the commit message.
+
+
+When filing a Pull Request, make sure it is rebased on top of most recent
+`master`. If you need to modify it or amend it in some way, you should always
+appropriately fixup the issues in git and force-push your changes to your fork.
+
+Also see: `Github Help: Using Pull Requests
+<https://help.github.com/articles/using-pull-requests/>`_
+
+
+Translations
+------------
+
+Please `translate online <http://pootle.locamotion.org/projects/pootle/>`_.
+
+
+Not sure how to contribute?
+---------------------------
+
+You can always seek for help in the `#pootle <irc://irc.freenode.net/#pootle>`_
+IRC channel.