This adds a qualified mention of golang/dep to the FAQ.

Fixes #19049

Change-Id: I42a114a008a6ca1250d849872dd98fd6523fa659
Reviewed-on: https://go-review.googlesource.com/46005Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>