ZOOKEEPER-3486: add the document about how to configure SSL/TLS for the admin server

- Link to the [PR-924](https://github.com/apache/zookeeper/pull/924)
- more details in the [ZOOKEEPER-3486](https://issues.apache.org/jira/browse/ZOOKEEPER-3486)

Author: maoling <maoling199210191@sina.com>

Reviewers: andor@apache.org

Closes #1036 from maoling/ZOOKEEPER-3486

zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md

@@ -2595,6 +2595,26 @@ The AdminServer is enabled by default, but can be disabled by either:
 Note that the TCP four-letter word interface is still available if
 the AdminServer is disabled.
 
+##### Configuring AdminServer for SSL/TLS
+- Generating the **keystore.jks** and **truststore.jks** which can be found in the [Quorum TLS](http://zookeeper.apache.org/doc/current/zookeeperAdmin.html#Quorum+TLS).
+- Add the following configuration settings to the `zoo.cfg` config file:
+
+```
+admin.portUnification=true
+ssl.quorum.keyStore.location=/path/to/keystore.jks
+ssl.quorum.keyStore.password=password
+ssl.quorum.trustStore.location=/path/to/truststore.jks
+ssl.quorum.trustStore.password=password
+```
+- Verify that the following entries in the logs can be seen:
+
+```
+2019-08-03 15:44:55,213 [myid:] - INFO  [main:JettyAdminServer@123] - Successfully loaded private key from /data/software/cert/keystore.jks
+2019-08-03 15:44:55,213 [myid:] - INFO  [main:JettyAdminServer@124] - Successfully loaded certificate authority from /data/software/cert/truststore.jks
+
+2019-08-03 15:44:55,403 [myid:] - INFO  [main:JettyAdminServer@170] - Started AdminServer on address 0.0.0.0, port 8080 and command URL /commands
+```
+
 Available commands include:
 
 * *connection_stat_reset/crst*: