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

Update #4582.

R=nigeltao, dave, adg
CC=golang-dev
https://golang.org/cl/7070065

doc/contribute.html

@@ -266,6 +266,8 @@ The special sentence “Fixes issue 159.” associates
 the change with issue 159 in the <a href="http://code.google.com/p/go/issues/list">Go issue tracker</a>.
 When this change is eventually submitted, the issue
 tracker will automatically mark the issue as fixed.
+(These conventions are described in detail by the
+<a href="http://code.google.com/p/support/wiki/IssueTracker#Integration_with_version_control">Google Project Hosting Issue Tracker documentation</a>.)
 </p>
 
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