Add contributor guidelines

CONTRIBUTING.md

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+# Contributing to Coinbase Node Client Library
+
+## Code of Conduct
+
+All interactions with this project follow our [Code of Conduct][code-of-conduct].
+By participating, you are expected to honor this code. Violators can be banned
+from further participation in this project, or potentially all Coinbase projects.
+
+[code-of-conduct]: https://github.com/coinbase/code-of-conduct
+
+## Bug Reports
+
+* Ensure your issue [has not already been reported][1]. It may already be fixed!
+* Include the steps you carried out to produce the problem.
+* Include the behavior you observed along with the behavior you expected, and
+  why you expected it.
+* Include any relevant stack traces or debugging output.
+
+## Feature Requests
+
+We welcome feedback with or without pull requests. If you have an idea for how
+to improve the project, great! All we ask is that you take the time to write a
+clear and concise explanation of what need you are trying to solve. If you have
+thoughts on _how_ it can be solved, include those too!
+
+The best way to see a feature added, however, is to submit a pull request.
+
+## Pull Requests
+
+* Before creating your pull request, it's usually worth asking if the code
+  you're planning on writing will actually be considered for merging. You can
+  do this by [opening an issue][1] and asking. It may also help give the
+  maintainers context for when the time comes to review your code.
+
+* Ensure your [commit messages are well-written][2]. This can double as your
+  pull request message, so it pays to take the time to write a clear message.
+
+* Add tests for your feature. You should be able to look at other tests for
+  examples. If you're unsure, don't hesitate to [open an issue][1] and ask!
+
+* Submit your pull request!
+
+## Support Requests
+
+For security reasons, any communication referencing support tickets for Coinbase
+products will be ignored. The request will have its content redacted and will
+be locked to prevent further discussion.
+
+All support requests must be made via [our support team][3].
+
+[1]: https://github.com/coinbase/coinbase-node/issues
+[2]: https://medium.com/brigade-engineering/the-secrets-to-great-commit-messages-106fc0a92a25
+[3]: https://support.coinbase.com/customer/en/portal/articles/2288496-how-can-i-contact-coinbase-support-