doc: add golanguage.ru to foreign-language doc list

R=r, brainman, r2
CC=golang-dev
https://golang.org/cl/3881041

doc/docs.html

@@ -79,6 +79,11 @@ An introduction to Go for C++ programmers.
 <li><a href="http://golang.jp/">golang.jp</a> - Go documentation and news.
 </ul>
 
+<h3 id="docs_ru">Russian &mdash; Русский</h3>
+<ul>
+<li><a href="http://golanguage.ru/">golanguage.ru</a> - Go documentation.
+</ul>
+
 </div>