HDFS-12695. Add a link to HDFS router federation document in site.xml. Contributed by Yiqun Lin.

hadoop-project/src/site/site.xml

@@ -103,6 +103,7 @@
       <item name="Disk Balancer" href="hadoop-project-dist/hadoop-hdfs/HDFSDiskbalancer.html"/>
       <item name="Upgrade Domain" href="hadoop-project-dist/hadoop-hdfs/HdfsUpgradeDomain.html"/>
       <item name="DataNode Admin" href="hadoop-project-dist/hadoop-hdfs/HdfsDataNodeAdminGuide.html"/>
+      <item name="Router Federation" href="hadoop-project-dist/hadoop-hdfs/HDFSRouterFederation.html"/>
     </menu>
 
     <menu name="MapReduce" inherit="top">