Create CONTRIBUTING.md (#102)

Signed-off-by: Andrew Lilley Brinker <alilleybrinker@gmail.com>

CONTRIBUTING.md

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+# Contributing
+
+We're happy to accept contributions!
+
+For bug fixes and minor changes to the implementation, feel free to open an issue
+in the issue tracker explaining what you'd like to fix, and then open a Pull
+Request with the change.
+
+For larger design changes, you may also want to discuss the changes either in the
+issue tracker or in the repository's Discussions page.
+
+## Developer Certificate of Origin
+
+Contributions to this repository are under the [Developer Certificate of Origin][dco]
+rules and are indicated by signing off on commits with the `-s`/`--signoff` flag
+when committing. This is enforced with a DCO CI job that checks your commits for
+signoff. If you forget to do it, the CI job gives instructions on how to fix
+your commits to include the signoff.
+
+[dco]: https://developercertificate.org/