Add contributing guide

CONTRIBUTING.md

+This Project is intended to be an open and friendly community. It doesn't matter whether you want to submit a bug or 
+feature request, improve code or documentation, provide a plugin or other assets - all contributions are welcome.
+
+This Project uses the the Collective Code Construction Contract ([C4]) as a basis for collaboration. Please familiarise
+yourself with these rules, and the additional rules below:
+
+* Every commit MUST be GPG-signed, either with the key of the Contributor (preferable) or the key of the Maintainer.
+
+To contribute to the code you should fork the [master branch on Gitlab](https://gitlab.com/solidninja/cryptsetup-rs).
+There is currently no project-specific style guide or development guide. Once you have a contribution, please open a 
+pull request. The patch shall be reviewed and merged.
+
+[c4]: https://rfc.zeromq.org/spec:42/C4/