Add a CONTRIBUTING document

Author: jleclanche

CONTRIBUTING.md

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+### Coding style
+
+Please adhere to the coding style throughout the project.
+
+1. Always use tabs. [Here](https://leclan.ch/tabs) is a short explanation why tabs are preferred.
+2. Always use double quotes for strings, unless single quotes avoid unnecessary escapes.
+3. When in doubt, [PEP8](https://www.python.org/dev/peps/pep-0008/). Follow its naming conventions.
+4. Know when to make exceptions.
+
+Also see: [How to name things in programming](http://www.slideshare.net/pirhilton/how-to-name-things-the-hardest-problem-in-programming)
+
+
+### Commits and Pull Requests
+
+Keep the commit log as healthy as the code. It is one of the first places new contributors will look at the project.
+
+1. No more than one change per commit. There should be no changes in a commit which are unrelated to its message.
+2. Every commit should pass all tests on its own.
+3. 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](https://help.github.com/articles/about-git-rebase/) 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/)