Adding documentation explaining Collections in the introductory Data sec...

Adding documentation explaining Collections in the introductory Data section, fixes #149. The worry with this ticket was that sample code uses Collections before explaining them, so someone reading the docs in order might get confused. 

This adds a few sentences and a code example declaring the Collections that are used in the next code sample. This just just a start after quickly reading the docs on Collections. Let me know if something needs to be restated.