added contributor guidelines to mirror what we have on readthedocs

CONTRIBUTING.md

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+# Contributing to Drone
+
+Want to hack on Drone? Awesome! Here are instructions to get you
+started. They are probably not perfect, please let us know if anything
+feels wrong or incomplete.
+
+## Contribution guidelines
+
+### Pull requests are always welcome
+
+We are always thrilled to receive pull requests, and do our best to
+process them as fast as possible. Not sure if that typo is worth a pull
+request? Do it! We will appreciate it.
+
+If your pull request is not accepted on the first try, don't be
+discouraged! If there's a problem with the implementation, hopefully you
+received feedback on what to improve.
+
+We're trying very hard to keep Drone lean and focused. We don't want it
+to do everything for everybody. This means that we might decide against
+incorporating a new feature.
+
+### Discuss your design on the mailing list
+
+We recommend discussing your plans [on the mailing
+list](https://groups.google.com/forum/?fromgroups#!forum/drone-dev)
+before starting to code - especially for more ambitious contributions.
+This gives other contributors a chance to point you in the right
+direction, give feedback on your design, and maybe point out if someone
+else is working on the same thing.
+
+We may close your pull request if not first discussed on the mailing
+list. We aren't doing this to be jerks. We are doing this to prevent
+people from spending large amounts of time on changes that may need
+to be designed or architected in a specific way, or may not align with
+the vision of the project.
+
+### Create issues...
+
+Any significant improvement should be documented as [a GitHub
+issue](https://github.com/drone/drone/issues) before anybody
+starts working on it.
+
+### ...but check for existing issues first!
+
+Please take a moment to check that an issue doesn't already exist
+documenting your bug report or improvement proposal. If it does, it
+never hurts to add a quick "+1" or "I have this problem too". This will
+help prioritize the most common problems and requests.