Reverb: Fill out README.md with the basics of how Reverb works.
Reverb: Fill out README.md with the basics of how Reverb works.

PiperOrigin-RevId: 310324392
Change-Id: I25b7ec80145fe9bcaa613a8713691d48fb7ad3e8

deepmind/reverb,deepmind/reverb,deepmind/reverb,deepmind/reverb