Create CONTRIBUTING.md (rrweb-io#969)

CONTRIBUTING.md

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+# Contributing to rrweb
+We want to make contributing to this project as easy and transparent as
+possible.
+
+## Our Development Process
+The majority of development on rrweb will occur through GitHub. Accordingly,
+the process for contributing will follow standard GitHub protocol.
+
+## Pull Requests
+We actively welcome your pull requests.
+1. Fork the repo and create your branch from `master`.
+2. If you've added code that should be tested, add tests
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Make sure your code lints and typechecks.
+
+## Issues
+We use GitHub issues to track public bugs. Please ensure your description is
+clear and has sufficient instructions to be able to reproduce the issue.
+
+## License
+rrweb is [MIT licensed](https://github.com/rrweb-io/rrweb/blob/master/LICENSE).
+
+By contributing to rrweb, you agree that your contributions will be licensed
+under its MIT license.