Add link to trusted tester sign-up mailing list post.
Add link to trusted tester sign-up mailing list post.

Change-Id: I7e4f9981ccf5bdf3d2828f6cd6cba42f0dd0b264

mzimmerman/appengine,flowlo/appengine,skyeunknown084/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,golang/appengine,mzimmerman/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,skyeunknown084/appengine,golang/appengine,skyeunknown084/appengine,flowlo/appengine