doc: link to Chinese translation of A Tour of Go

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5415046

doc/docs.html

@@ -130,6 +130,7 @@ Notable articles from the <a href="http://blog.golang.org/">Go Blog</a>.
 <h3 id="docs_cn">Chinese &mdash; 中文</h3>
 
 <ul>
+<li><a href="http://go-tour-zh.appspot.com/">A Tour of Go</a></li>
 <li><a href="http://code.google.com/p/golang-china/">golang-china</a> - a broad range of Go documentation.</li>
 <li><a href="http://code.google.com/p/ac-me/downloads/detail?name=fango.pdf">Effective Go and Tutorial</a></li>
 </ul>