Polish docs a little

doc/folder_structure.md:
* Be specific about where links are going
* Say what the bare minimum is and what we do on top of it
* .tar.bz2 is now an option

doc/getting_started.md
* Add a 'prerequisites' section
* Link to all useful context for people new to packaging
Merge pull request #19 from rgarner/docs

Polish docs a little