meta: add CONTRIBUTING.md

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>

CONTRIBUTING.md

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+Thank you for your interest in contributing to this project!
+
+Before you submit your first patch, please take a moment to review the following
+guidelines:
+
+## Certificate of Origin
+
+By contributing to this project, you agree to the [Developer Certificate of
+Origin (DCO)][dco]. This document was created by the Linux Kernel community
+and is a simple statement that you, as a contributor, have the legal right to
+make the contribution.
+
+In order to show your agreement with the DCO you should include at the end of
+the commit message, the following line:
+
+    Signed-off-by: John Doe <john.doe@example.com>
+
+using your real name and email.
+
+This can be done easily using `git commit -s`.
+
+### Fixing the DCO
+
+If you did not sign-off one or more of your commits then it is not all for not.
+The crowd at `src-d` have a [wonderful guide][fixing-dco] on how to remedy this
+situation.
+
+## License Header
+
+As part of our license, we must include a license header at the top of each
+source file. The template for this header can be found [here][header-template].
+If you are creating a new file in the repository you will have to add this
+header to the file.
+
+[dco]: ./DCO
+[fixing-dco]: https://github.com/src-d/guide/blob/master/developer-community/fix-DCO.md
+[header-template]: ./.license-header