Adding Contributing file (#128)

Summary:

This is pull request was created automatically because we noticed your project was missing a Contributing file.

CONTRIBUTING files explain how a developer can contribute to the project - which you should actively encourage.

This PR was crafted with love by Meta's Open Source Team.


Differential Revision: D67979754

Pulled By: sdruzkin

Author: facebook-github-bot

CONTRIBUTING.md

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+# Contributing to nimble
+We want to make contributing to this project as easy and transparent as
+possible.
+
+## Pull Requests
+We actively welcome your pull requests.
+
+1. Fork the repo and create your branch from `main`.
+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.
+6. If you haven't already, complete the Contributor License Agreement ("CLA").
+
+## Contributor License Agreement ("CLA")
+In order to accept your pull request, we need you to submit a CLA. You only need
+to do this once to work on any of Facebook's open source projects.
+
+Complete your CLA here: <https://code.facebook.com/cla>
+
+## 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.
+
+Meta has a [bounty program](https://bugbounty.meta.com/) for the safe
+disclosure of security bugs. In those cases, please go through the process
+outlined on that page and do not file a public issue.
+
+## License
+By contributing to Nimble, you agree that your contributions will be licensed
+under the LICENSE file in the root directory of this source tree.