Refer to Zookeeper license in master (#868)

* Update note on exposing ports

* Notes on the new environment variables

* `/conf` is not a volume

* Enhance documentation

* Refer to Zookeeper license in master

zookeeper/license.md

@@ -1 +1 @@
-View [license information](https://github.com/apache/zookeeper/blob/release-3.4.9/LICENSE.txt) for the software contained in this image.
+View [license information](https://github.com/apache/zookeeper/blob/master/LICENSE.txt) for the software contained in this image.