Add initial CONTRIBUTING document

Based on
https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

Ref #287

CONTRIBUTING.rst

@@ -0,0 +1,77 @@
+*****************
+How to contribute
+*****************
+
+So you want to contribute to the Powerline project? Awesome! This document 
+describes the guidelines you should follow when making contributions to the 
+project.
+
+**Please note that these guidelines aren't mandatory in any way, but your 
+pull request will be merged a lot faster if you follow them.**
+
+Getting started
+===============
+
+* Make sure you have a `GitHub account <https://github.com/signup/free>`_.
+* Submit an `issue on GitHub 
+  <https://github.com/Lokaltog/powerline/issues>`_, assuming one does not 
+  already exist.
+
+  * Clearly describe the issue.
+  * If the issue is a bug: Make sure you include steps to reproduce, and 
+    include the earliest revision that you know has the issue.
+
+* Fork the repository on GitHub.
+
+Making changes
+==============
+
+* Create a topic branch from where you want to base your work.
+
+  * Powerline uses the `Git Flow 
+    <http://nvie.com/posts/a-successful-git-branching-model/>`_ branching 
+    model.
+  * Most contributions should be based off the ``develop`` branch.
+  * Prefix your branch with ``feature/`` if you're working on a new feature.
+  * Include the issue number in your topic branch, e.g.  
+    ``321-fix-some-error`` or ``feature/123-a-cool-feature``.
+
+* Make commits of logical units.
+* Run your code through ``flake8`` and fix any programming style errors. Use 
+  common sense regarding whitespace warnings, not all warnings need to be 
+  fixed.
+* Make sure your commit messages are in the `proper format 
+  <http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html>`_.  
+  The summary must be no longer than 70 characters. Refer to any related 
+  issues with e.g. ``Ref #123`` or ``Fixes #234`` at the bottom of the 
+  commit message. Commit messages can use Markdown with the following 
+  exceptions:
+
+  * No HTML extensions.
+  * Only indented code blocks (no ``````` blocks).
+  * Long links should be moved to the bottom if they make the text wrap or 
+    extend past 72 columns.
+
+* Make sure you have added the necessary tests for your changes.
+* Run *all* the tests to assure nothing else was accidentally broken.
+
+Programming style
+-----------------
+
+* The project uses *tabs for indentation* and *spaces for alignment*, this 
+  is also included in a vim modeline on top of every script file.
+* Run your code through ``flake8 
+  --ignore=W191,E501,E121,E122,E123,E128,E225`` to fix any style errors. Use 
+  common sense regarding whitespace warnings, not all ``flake8`` warnings 
+  need to be fixed.
+* Trailing whitespace to indicate a continuing paragraph is OK in comments, 
+  documentation and commit messages.
+
+Submitting changes
+==================
+
+* Push your changes to a topic branch in your fork of the repository.
+* If necessary, use ``git rebase -i <revision>`` to squash or reword commits
+  before submitting a pull request.
+* Submit a pull request to `Lokaltog's repository 
+  <https://github.com/Lokaltog/powerline>`_.