Update readme for recent changes

Summary: Also add a note about how clock ids are recommended for advanced users.

Test Plan: Read the readme.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D903941

--HG--
extra : rebase_source : 5abd23513fa67114be97ab0b16883c39a1ba3aa5
extra : amend_source : 3da291c215dba6fa2a9220eb6f828ec8e6fd289a