[SR] Update README (#75283)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75283
Update the `README` with more implementation and usage details. The descriptions are mostly high level, but it aims to cover all of the FAQs we've seen from new team members.
ghstack-source-id: 153292076
Test Plan: N/A
Reviewed By: navahgar
Differential Revision: D35330431
fbshipit-source-id: 93040fb55bb842ccee348759ba91200c3d9e80b8
(cherry picked from commit 1346ce915aead69008eb6006e9396843a60fcf82)
