Define root README.md for technical docs (#50333)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50333

Changelog: [internal]

This defines a placeholder for the main entrypoint for the React Native internal technical documentation.

Reviewed By: javache, mdvacca, rshest

Differential Revision: D71889755

fbshipit-source-id: 453518a0b7ec2cf433884a87446a7a622418d0b2