doc/contribute: Link to IssueTracker docs for more "special sentences"
doc/contribute: Link to IssueTracker docs for more "special sentences"

Update issue 4582.

R=nigeltao, dave, adg
CC=golang-dev
https://codereview.appspot.com/7070065

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go