Update README.md w/ USE_JAVA_VERSION envvar

`USE_JAVA_VERSION` env var allows the user to set the java version of the jnlp client running in the jenkins agent image.

README.md

@@ -171,6 +171,7 @@ initialization by passing `-e VAR=VALUE` to the Docker run command.
 |  `MAVEN_SLAVE_IMAGE`   | Setting this value overrides the image used for the default maven agent pod configuration.  For 3.x, the default maven agent pod uses `docker.io/openshift/jenkins-agent-maven-35-centos7` or `registry.redhat.io/openshift3/jenkins-agent-maven-35-rhel7` depending whether you are running the centos or rhel version of the Jenkins image.  For 4.x, the image is included in the 4.0 payload via an imagestream in the openshift namespace, and the image spec points to the internal image registry.  If you are running this image outside of OpenShift, you must either set this environment variable or manually update the setting to an accessible image spec. This variable must be set before Jenkins starts the first time for it to have an effect. |
 |  `JENKINS_UC_INSECURE`       | When your Jenkins Update Center repository is using a self-signed certificate with an unknown Certificate Authority, this variable allows one to bypass the repository's SSL certificate check. The variable applies to download of the plugin which may occur during Jenkins image build, if you build an extension of the jenkins image or if you run the Jenkins image and leverage one of the options to download additional plugins (use of s2i whith plugins.txt or use of `INSTALL_PLUGINS` environment variable. |
 | `MAVEN_MIRROR_URL` | Allows you to specify a [Maven mirror repository](https://maven.apache.org/guides/mini/guide-mirror-settings.html) in the form of `MAVEN_MIRROR_URL=https://nexus.mycompany.com/repository/maven-public`. The mirror repository is used by maven as an additional location for artifacts downloads. For more details on [how maven mirrors works](https://maven.apache.org/guides/mini/guide-mirror-settings.html), you can refer to maven documentation.
+| `USE_JAVA_VERSION`	|	Allows you to set the Java version used by the JNLP Client for the agent images. By default the value is set to `java-11`	|
 
 You can also set the following mount points by passing the `-v /host:/container` flag to Docker.