Move, rename, and document Visual Studio GN helper script

BUG=skia:

DOCS_PREVIEW= https://skia.org/?cl=5983

Change-Id: I32ac2c5275eb86adaa2be9ceee061b2956db70ad
Reviewed-on: https://skia-review.googlesource.com/5983
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
2 files changed