Updates to installation instructions for Virtual Machine and Docker images

Staged at http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.0/en/getstarted.html
Listed and created sections for the three different download methods.
Updated instructions for Virtual Machine and Docker.
and using the zip file is not easy? To me it is the easiest, because I don't need to download a VM. And I have the prereqs installed anyway. I believe so do most developers. The VM is easier for less technical people who don't have a development environment. The docker image is the hardest to set up, and I think it really makes it easy for people who develop on Linux, but it requires the most technical understanding, about port forwarding in Docker and VirtualBox etc if you run on MacOS.