HBASE-711 Complain if clock skew across the cluster is badly out of sync; Documentation

git-svn-id: https://svn.apache.org/repos/asf/hadoop/hbase/trunk@678223 13f79535-47bb-0310-9956-ffa450edef68

src/java/overview.html

@@ -40,6 +40,9 @@
   amount of data into regionservers.  See the
   <a href="http://wiki.apache.org/hadoop/Hbase/FAQ#6">FAQ: Why do I see "java.io.IOException...(Too many open files)" in my logs?</a>
   for how to up the limit.</li>
+      <li>The clocks on cluster members should be in basic alignments.  Some skew is tolerable but
+      wild skew can generate odd behaviors.  Run <a href="http://en.wikipedia.org/wiki/Network_Time_Protocol">NTP</a>
+      on your cluster, or an equivalent.</li>
 </ul>
 
 <h2><a name="getting_started" >Getting Started</a></h2>