Add rollback user guide as README.md

This takes the internal rollback user guide
(go/rollback-manager-user-guide), updates the content, and moves it into
the codebase to make this information available alongside the
RollbackManager code.

Bug: 132758097
Test: N/A
Change-Id: I9c15fca1c4f5c29266ad271b0e3806661c7cc327