Update and expand IdentityManager README.md

Bug: 970295
Change-Id: I09a0a335b324a8b321b5f8deed3de5933d889118
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724683
Reviewed-by: Lowell Manners <lowell@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682275}
1 file changed