doc: link to design doc for GOMAXPROCS change in go1.5.html

Change-Id: Ifac10621fece766f3a0e8551e98d1f8d7072852f
Reviewed-on: https://go-review.googlesource.com/13068Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.html

@@ -199,6 +199,8 @@ Programs that do not expect to run with multiple cores may
 break inadvertently.
 They can be updated by removing the restriction or by setting
 <code>GOMAXPROCS</code> explicitly.
+For a more detailed discussion of this change, see
+the <a href="https://golang.org/s/go15gomaxprocs">design document</a>.
 </p>
 
 <h3 id="build">Build</h3>