We had some issues with reports being marked as spam, so I added a
filter to never mark as spam something that mentions the word
"vulnerability". We get too much spam at that address to disable the
filter entirely, so instead meantion the bypass in the docs.

Change-Id: Idb4dabcf51a9dd8234a2d571cd020c970b0a582c
Reviewed-on: https://go-review.googlesource.com/c/go/+/205538Reviewed-by: Katie Hockman <katie@golang.org>