Adds info from current landing page to README file Also adds a table to document support for various APIs and platforms. Change-Id: Ie032f553dec23516242a96ce6788fb086fad4229 Reviewed-on: https://chromium-review.googlesource.com/311021 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Shannon Woods <shannonwoods@chromium.org>