adding docker info to readme

The npm install and git pull command for virgo is probably wrong. It did not work for me in the given format.

Also adding information to build using docker.
This command gives me usage instructions:

C02LV0QBFH04:rackspace-monitoring-js eugeniag$ docker build -t .

Usage: docker build [OPTIONS] PATH | URL | -

Build a new image from the source code at PATH

  --force-rm=false     Always remove intermediate containers, even after unsuccessful builds
  --no-cache=false     Do not use cache when building the image
  -q, --quiet=false    Suppress the verbose output generated by the containers
  --rm=true            Remove intermediate containers after a successful build
  -t, --tag=""         Repository name (and optionally a tag) to be applied to the resulting image in case of success