docs: add dynamic configuration doc

fixes #1418 
reference:http://www.slideshare.net/Hadoop_Summit/dynamic-reconfiguration-of-zookeeper
https://docs.google.com/document/d/1AF8pIfQbN5cKxe0c4cQ4_DW6ZjBJqSkyANcTGUwkzjc

We need to mention etcd supports incremental configuration changes.