doc: A Tour of Go in Japanese

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5318058

doc/docs.html

@@ -157,6 +157,7 @@ Notable articles from the <a href="http://blog.golang.org/">Go Blog</a>.
 
 <h3 id="docs_jp">Japanese &mdash; 日本語</h3>
 <ul>
+<li><a href="http://go-tour-jp.appspot.com/">A Tour of Go</a></li>
 <li><a href="http://golang.jp/">golang.jp</a> - Go documentation and news.</li>
 </ul>