Add Contributing.md (#2699)

* Fix review nits

CONTRIBUTING.md

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+Contributing
+============
+
+This is the code for the Mozilla Add-ons website frontend–it's the code that displays Add-ons to user from the Add-ons (API) Server. This project handles the display of data and submission of things like reviews to the API, but doesn't handle the data. If you’re not sure this is the correct place to file an issue then please file an issue on the [mozilla/addons] project instead.
+
+Before contributing, please note:
+
+* You agree to license your contributions under the [license].
+* Please file issues or ask on the [dev-addons mailing list] before submitting pull-requests for new features or large changes that are not related to existing issues.
+* Please write tests for each patch to maintain test coverage.
+* Please read the [README] for instructions on how to use the repo.
+
+Ready to get started? Follow [these steps].
+
+Thank you for contributing!
+
+## Note to staff
+
+If you come across a potential `good first bug` for contributors, please tag it with `maybe good first bug`. The community team [triages] these every other week to ensure they have mentors assigned, onboarding information, and basic steps to get started. This gives new contributors a better experience when they pick a “good first bug” to work on.
+
+  [mozilla/addons]: https://github.com/mozilla/addons/issues/new
+  [license]: https://github.com/mozilla/addons-frontend/blob/master/LICENSE
+  [dev-addons mailing list]: https://mail.mozilla.org/listinfo/dev-addons
+  [README]: https://github.com/mozilla/addons-frontend/blob/master/README.md
+  [these steps]: https://wiki.mozilla.org/Add-ons/Contribute/Code
+  [triages]: https://wiki.mozilla.org/Add-ons/Contribute/Goodfirstbugs_triage