RFC: A new Cabal user guide

[ketzacoatl]

@FrancisKing

The first item has got to be - what is Cabal, and how does it relate to Stack. Then an explanation of the two kinds of Cabal, v1 and v2, and what the difference is. I got into difficulties with the last point.

This would be really nice to clarify for new haskellers beginning their journey!