doc/go1.14: mention update to Unicode 12

Updates #36878

Change-Id: Ia8441de8a04fead1f76c72f662bcb877e2265c3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/217133
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: ianlancetaylor

doc/go1.14.html

@@ -663,6 +663,17 @@ <h2 id="library">Core library</h2>
     </p>
 </dl><!-- testing -->
 
+<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 Unicode 11.0 to
+      <a href="https://www.unicode.org/versions/Unicode12.0.0/">Unicode 12.0</a>,
+      which adds 554 new characters, including four new scripts, and 61 new emoji.
+    </p>
+  </dd>
+</dl><!-- unicode -->
+
 <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <p>