Add CONTRIBUTING (#78)

CONTRIBUTING.md

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+# Contributing Guide
+
+## Code of Conduct
+
+We welcome pull requests from everyone. By participating in this project, you
+agree to abide by the thoughtbot [code of conduct].
+
+We expect everyone to follow the code of conduct anywhere in thoughtbot's
+project codebases, issue trackers, chat-rooms, and mailing lists.
+
+[code of conduct]: https://thoughtbot.com/open-source-code-of-conduct
+
+## Getting Started
+
+`administrate-field-nested_has_many` is a field plugin for [Administrate][],
+that allows for nested `has_many` forms.
+
+To maintain compatibility with different dependency versions, we use
+[Appraisal][].
+
+[Administrate]: https://github.com/thoughtbot/administrate
+[Appraisal]: https://github.com/thoughtbot/appraisal
+
+### Opening a PR
+
+1. Fork the repo,
+2. Run `./bin/setup` to install the base dependencies and setup a local
+   database,
+3. Run the test suite: `bundle exec rspec && bundle exec appraisal rspec`,
+4. Make your changes,
+5. Push your fork and open a pull request.
+
+A good PR will solve the smallest problem it possibly can, have good test
+coverage and (where necessary) have internationalisation support.
+
+## Security
+
+See the [security policy](./SECURITY.md).