Add contributing guidelines (#458)

CONTRIBUTING.md

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+# Contributing to Abacus
+
+We'd love for you to contribue to Abacus! Here are the guidelines to follow when contributing:
+
+* [Code of Conduct](#coc)
+* [Questions and Problems](#question)
+* [Issue Submission Guidelines](#submit)
+* [Pull Request Submission Guidelines](#submit-pr)
+
+## <a name="coc"></a> Code of Conduct
+
+Help us keep Abacus open and inclusive. Please read and follow our [Code of Conduct][coc].
+
+## <a name="question"></a> Got a Question or Problem?
+
+Please do not open issues for general questions as we want to keep GitHub issues for bug reports
+and feature requests. You've got much better chances of getting your question answered on
+[our Discord Server][discord].
+
+## <a name="submit"></a> Issue Submission Guidelines
+
+Please submit a [new issue][github-new-issue] if you have a bug report or feature request.
+
+Help us to maximize the effort we can spend fixing issues and adding new features by not reporting duplicate issues.
+
+## <a name="submit-pr"></a> Pull Request Submission Guidelines
+Thank you for considering to help out with the source code! We welcome contributions
+from anyone on the internet, and are grateful for even the smallest of fixes!
+
+If you'd like to contribute to Abacus, please fork, fix, commit and send a pull request
+for the maintainers to review and merge into the main code base. If you wish to submit
+more complex changes, please check up with the core devs first on [our Discord Server][discord]
+to ensure those changes are in line with the general philosophy of the project and/or get
+some early feedback which can make both your efforts much lighter as well as our review
+and merge procedures quick and simple.
+
+[coc]: https://github.com/abacus-network/abacus-monorepo/blob/main/CODE_OF_CONDUCT.md
+[github-new-issue]: https://github.com/abacus-network/abacus-monorepo/issues/new
+[discord]: https://discord.com/invite/KBD3aD78Bb