Only exact version of Windows SDK works.
Clarify how to do a release build.

BUG=angleproject:2297

Change-Id: I2566b7e807a84c4edc3b457299036e67a64b5a36
Reviewed-on: https://chromium-review.googlesource.com/852553
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>