Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41706

This change introduces a ReadME in the CircleCI folder.
This can be used as documentation to learn more about our CircleCI setup and will also help GitHub employees in executing the migration.

## Changelog:
[Internal] - Add CircleCI documentation

Reviewed By: cortinico

Differential Revision: D51665453

fbshipit-source-id: f61325ed26572c4a8d4a68db1cca5934d3d968fb