Update docs to mention Win 10 SDK is required. Uncovered this after reports to the Google group. Also fix a minor documentation issue with Windows store. BUG=angleproject:1944 BUG=angleproject:1255 Change-Id: Ib4fc784a818cf65e280630db483987cc01366994 Reviewed-on: https://chromium-review.googlesource.com/657881 Reviewed-by: Jamie Madill <jmadill@chromium.org>