Add CONTRIBUTING.md file

Adding a CONTRIBUTING.md file so that contributors are prompted to
look at the file when making a PR.

Signed-off-by: Fintan Halpenny <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+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 <[email protected]>
+
+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.
+
+## Modifying the specs
+
+When any of the spec files is modified, e.g. one from `docs/spec/sections`,
+the docs need to be re-rendered and the files in `docs/spec/out` need to be
+updated. To do this, run `scripts/render-docs`.
+
+[dco]: ./DCO
+[fixing-dco]: https://docs.github.com/en/free-pro-team@latest/github/building-a-strong-community/creating-a-pull-request-template-for-your-repository
+[header-template]: ./.license-header-template
+