Update documentation about adding workers from images: warn about having Java installed

As the connection from master to worker is using SSH, I'd see it very convenient to inform the users that Java must be installed in the image used to build the workers. I noticed that the SSH-slaves plugin already does that: https://github.com/jenkinsci/ssh-slaves-plugin/blob/master/doc/CONFIGURE.md#prerequisites:

>Before you start to use the SSH Slaves Plugin with your agents, you need to prepare your agents to run the remoting process. This means that you have to install a JDK/JRE 8 on your agent in order to run the remoting.jar process.

I simple warning message before the image select input could be enough.