doc: update 1.7 release notes on Unicode upgrade

Fixes #16201

Change-Id: I38c17859db78c2868905da24217e0ad47739c320
Reviewed-on: https://go-review.googlesource.com/24541
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.7.html

@@ -1222,3 +1222,15 @@ system call before executing the new program.
 </p>
 </dd>
 </dl>
+
+
+<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
+
+<dd>
+<p>
+The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
+support throughout the system has been upgraded from version 8.0 to
+<a href="http://www.unicode.org/versions/Unicode9.0.0/">Unicode 9.0</a>.
+</p>
+</dd>
+</dl>