Add documentation for Viewer

BUG=skia:5488

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=5682

Change-Id: I635468f6a17576e990e159f7f02ba2f1946317ab
Reviewed-on: https://skia-review.googlesource.com/5682
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed