Re-add the top level LICENSE file (#37916)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37916

Repos on GitHub should have a top level LICENSE file to make sure GitHub can autodiscover
the correct license for the project.

Once we moved the file inside packages/react-native/ the autodiscovery is not working anymore (it returns a 404).

Changelog:
[Internal] - Re-add the top level LICENSE file

Reviewed By: hoxyq

Differential Revision: D46764347

fbshipit-source-id: cd760c004cf97248490b43f7ba08a47ef2d23c89