Add basics of accessing a command prompt on OSX and Windows

[daguar]

I'm finding I need this for the CityVoice documentation — where non-technical users can easily deploy to Heroku without anything beyond the toolbelt, but they do need to know how to access a command prompt.

So I'm adding a stub; might do myself, but would love to know if this scratches someone else's itch.

[shannonzylstra]

Is help still needed to write documentation for accessing the Windows Command Prompt?

[paulb777]

http://blog.teamtreehouse.com/introduction-to-the-mac-os-x-command-line