Merge pull request #2032 from xiang90/note

doc: add note for reconfiguration

Documentation/2.0/runtime-configuration.md

@@ -2,6 +2,10 @@
 
 etcd comes with support for incremental runtime reconfiguration, which allows users to update the membership of the cluster at run time.
 
+Reconfiguration requests can only be processed when the the majority of the cluster members are functioning. It is **highly recommended** to always have a cluster size greater than two in production. It is unsafe to remove a member from a two member cluster. The majority of a two member cluster is also two. If there is a failure during the removal process, the cluster might not able to make progress and need to [restart from majority failure][majority failure].
+
+[majority failure]: #restart-cluster-from-majority-failure
+
 ## Reconfiguration Use Cases
 
 Let us walk through the four use cases for re-configuring a cluster: replacing a member, increasing or decreasing cluster size, and restarting a cluster from a majority failure.