Adding references about agent port configuration to README

README.md

@@ -35,7 +35,7 @@ NOTE: Avoid using a [bind mount](https://docs.docker.com/storage/bind-mounts/) f
 docker run -d -v jenkins_home:/var/jenkins_home -p 8080:8080 -p 50000:50000 jenkins/jenkins:lts
 ```
 
-this will run Jenkins in detached mode with port forwarding and volume added. You can access logs with command 'docker logs CONTAINER_ID' in order to check first login token. ID of container will be returned from output of command above. 
+this will run Jenkins in detached mode with port forwarding and volume added. You can access logs with command 'docker logs CONTAINER_ID' in order to check first login token. ID of container will be returned from output of command above.
 
 ## Backing up data
 
@@ -106,7 +106,7 @@ If you want to install Jenkins behind a reverse proxy with prefix, example: mysi
 
 # Passing Jenkins launcher parameters
 
-Argument you pass to docker running the jenkins image are passed to jenkins launcher, so you can run for sample :
+Argument you pass to docker running the jenkins image are passed to jenkins launcher, so you can run for sample:
 ```
 docker run jenkins/jenkins:lts --version
 ```
@@ -136,6 +136,12 @@ or as a parameter to docker,
 docker run --name myjenkins -p 8080:8080 -p 50001:50001 --env JENKINS_SLAVE_AGENT_PORT=50001 jenkins/jenkins:lts
 ```
 
+**Note**: This environment variable will be used to set the port using the
+[system property][system-property] `jenkins.model.Jenkins.slaveAgentPort` in **JAVA_OPTS**.
+
+> If this property is already set in **JAVA_OPTS**, then the value of
+`JENKINS_SLAVE_AGENT_PORT` will be ignored.
+
 # Installing more tools
 
 You can run your container as root - and install via apt-get, install as part of build steps via jenkins tool installers, or you can create your own Dockerfile to customise, for example:
@@ -272,3 +278,5 @@ If you wish to contribute fixes to this repository, please refer to the [dedicat
 # Questions?
 
 Jump on irc.freenode.net and the #jenkins room. Ask!
+
+[system-property]: https://wiki.jenkins.io/display/JENKINS/Features+controlled+by+system+properties