Related to #442 - Some changes to Quick-Start guide: add $ signs and clarify Installation for Development

I added a few sentences to the Installation for Development section, putting the link to the Github instructions in an introductory paragraph rather than hidden at the end. I have also put a $ at the beginning of each command line instruction for the sake of consistency.
"Pip" should be "pip" (see https://github.com/pypa/pip).