docs: Add contribution guidelines

* High level overview of contribution guidelines.

CONTRIBUTING.md

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+# Contributing to docker-openvpn
+
+Community contributions are welcome and help move the project along.  Please review this document before sending any pull requests.
+
+Thanks!
+
+## Bug Fixes
+
+All bug fixes are welcome.  Please try to add a test if the bug is something that should have been fixed already.  Oops.
+
+## Feature Additions
+
+New features are welcome provided that the feature has a general audience and is reasonably simple.  The goal of the repository is to support a wide audience and be simple enough.
+
+Please add new documentation in the `docs` folder for any new features.  Pull requests for missing documentation is welcome as well.  Keep the `README.md` focused on the most popular use case, details belong in the docs directory.
+
+If you have a special feature, you're likely to try but it will likely be rejected if not too many people seem interested.
+
+## Tests
+
+In an effort to not repeat bugs (and break less popular features), unit tests are run on [Travis CI](https://travis-ci.org/kylemanna/docker-openvpn).  The goal of the tests are to be simple and to be placed in the `tests` directory where it will be automatically run.  Review existing tests for an example.
+
+## Style
+
+The style of the repo follows that of the Linux kernel, in particular:
+
+* Pull requests should be rebased to small atomic commits so that the merged history is more coherent
+* The subject of the commit should be in the form "<subsystem>: <subject>"
+* More details in the body
+* Match surrounding coding style (line wrapping, spaces, etc)
+
+More details in the [SubmittingPatches](https://www.kernel.org/doc/Documentation/SubmittingPatches) document included with the Linux kernel.  In particular the following sections:
+
+* `2) Describe your changes`
+* `3) Separate your changes`