Add simple CONTRIBUTING doc

CONTRIBUTING.md

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+Thanks for wanting to contribute to Mozilla's Add-ons Validator! You rock! 😊
+
+## Submitting a Pull Request
+
+If you're submitting a pull request, please reference the issue it closes
+inside your pull request. Keep your commits atomic; you should have one
+commit per issue solved.
+
+Add tests for your code and make sure all existing tests pass. If the tests
+fail or you don't maintain 100% test coverage we won't be able to accept your
+pull request.
+
+### Tests
+
+Our tests include `eslint` and `jscs` style checks; these keep our code
+consistent. You can review the rules in the [`.eslintrc`][eslint] and
+[`.jscsrc`][jscsrc] files.
+
+Please run the tests locally with `npm test` before you commit.
+
+[eslint]: https://github.com/mozilla/addons-validator/blob/master/.eslintrc
+[jscsrc]: https://github.com/mozilla/addons-validator/blob/master/.jscsrc