You must sign a Contribution License Agreement (CLA) before your pull request can be merged. This a one-time requirement for Meta projects in GitHub. You can read more about Contributor License Agreements (CLA) on Wikipedia.
However, you don't have to do this up-front. We welcome you to follow, fork, and submit a pull request.
When your pull request is created, it is classified by the Facebook GitHub bot. If you have not signed a CLA, the bot will provide instructions for signing a CLA before your pull request can be considered eligible for merging. Once you have done so, the current and all future pull requests will be labelled as "CLA Signed".
Signing the CLA might sound scary, but it's actually very simple and can be done in less than a minute.