add docker information

Hi there,

I'm not sure, if the "Getting Started" section is the correct place, but i added the docker information near to the Vagrant part. Another possibility could be moving Vagrant and Docker to the "Servers And Deployment" section, but this should be an additional PR, imho.

Please let me know if the provided information is enough, or should cover more details.
I thought it would be helpful enough to point people to the existence of docker and give a *very* short introduction of what it is and how it can be used.

best regards,
Matthias