Add initial contributing doc

CONTRIBUTING.md

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+# Contributing
+
+## How to Contribute
+
+#### Code of Conduct
+
+This repository has adopted the Contributor Covenant as it's
+Code of Conduct. It is expected that participants adhere to it.
+
+#### Proposing a Change
+
+If you are unsure about whether or not a change is desired,
+you can create an issue. This is useful because it creates
+the possibility for a discussion that's visible to everyone.
+
+When fixing a bug it is fine to submit a pull request right away.
+
+#### Sending a Pull Request
+
+Steps to be performed to submit a pull request:
+
+1. Fork the repository and create your branch from `master`.
+2. Run `yarn` in the repository root.
+3. If you've fixed a bug or added code that should be tested, add tests!
+4. Fill out the description, link any related issues and submit your pull request.
+
+#### Pull Request Prerequisites
+
+You have [Node](https://nodejs.org/) installed at v12.2.0+ and [Yarn](https://yarnpkg.com/) at v1.18.0+.
+
+Please note that commit hooks will run automatically to perform some tasks;
+
+- format your code
+- run tests
+- build distributable files
+
+#### License
+
+By contributing to this repository, you agree that your contributions will be licensed under its MIT license.