Fix README.md for commonmark with small edits Change-Id: Id7ed37945744a083e6eb5b6a792f8379b7b98bac Reviewed-on: https://chromium-review.googlesource.com/348200 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>