We no longer require a specific version of the Windows SDK.
Also include updated instructions on how to setup Goma.

Bug: angleproject:1944
Change-Id: I5f9e714ebd69ba04eddc1b1fc2f95d1fde67a47f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2522419Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>