Add a link to a basic command-line primer to the Getting Started page

Grunt got called out (in a friendly way) at WordCamp Boston for making the assumption in its docs that people know command-line basics. While this is not an _atrociously_ unsafe assumption, there's definitely an audience of beginning developers who are less familiar with the command line, across both Windows and OSX platforms. We ought to make a note in getting started that Grunt is first and foremost a CLI, and link to a quick primer on how the command line works in different environments.

I'm vetting primers now, but I'm not sure where the content itself actually lives in this repo so I'm not yet able to PR.