doc: add nodejs.vn to community page

Node.js.  It's not just for SF hipsters anymore.

doc/community/index.html

@@ -200,6 +200,7 @@
               <a class="spain" href="http://www.nodejs.es">Node.js Español</a> Node.js in Spanish<br>
               <a class="taiwain" href="http://nodejs.tw">nodejs.tw</a> Taiwan community<br>
               <a class="turkey" href="http://www.nodejstr.com/">Node.js Türkiye</a> Node.js in Turkish<br>
+              <a class="vietnam" href="http://nodejs.vn/">Node.js Viet Nam</a> Node.js in Vietnamese<br>
             </p>
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         </div>