Add CONTRIBUTING.md

**what is the change?:**
Even though this is a small project and we don't expect changes, it's
standard to include a `CONTRIBUTING.md` doc.

**why make this change?:**
Let's give folks info about how to contribute, to foster a welcoming
open source community in the React ecosystem.

**test plan:**
Viewing this file on my branch.

**issue:**
T23481323

CONTRIBUTING.md

@@ -0,0 +1,34 @@
+# Contributing to prop-types
+We want to make contributing to this project as easy and transparent as
+possible.
+
+## Code of Conduct
+The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md).
+
+## 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.
+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.
+
+Facebook has a [bounty program](https://www.facebook.com/whitehat/) 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 prop-types, you agree that your contributions will be licensed
+under the LICENSE file in the root directory of this source tree.