Add contributing.md (#252)

.github/CONTRIBUTING.md

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+## Contributing
+
+[legal]: https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license
+[license]: ../LICENSE
+[code-of-conduct]: CODE-OF-CONDUCT.md
+
+Hi! Thanks for your interest in contributing to the GitHub CLI!
+
+Given that this project is very early and still in beta, we're only accepting pull requests for bug fixes right now. We'd love to
+hear about ideas for new features as issues, though!
+
+Please do:
+
+* open an issue if things aren't working as expected
+* open an issue to propose a significant change
+* open a PR to fix a bug
+
+## Submitting a bug fix
+
+0. Clone this repository
+0. Create a new branch: `git checkout -b my-branch-name`
+0. Make your change, add tests, and ensure tests pass
+0. Make a PR: `gh pr create --web`
+
+Contributions to this project are [released][legal] to the public under the [project's open source license][license].
+
+Please note that this project adheres to a [Contributor Code of Conduct][code-of-conduct]. By participating in this project you agree to abide by its terms.
+
+## Resources
+
+- [How to Contribute to Open Source](https://opensource.guide/how-to-contribute/)
+- [Using Pull Requests](https://help.github.com/articles/about-pull-requests/)
+- [GitHub Help](https://help.github.com)