Updated README.md sandbox instructions for use with cabal 1.20.

The README could do with some updated instructions for how to use the Cabal sandbox. [Google group discussion](https://groups.google.com/forum/#!topic/yi-devel/hYvT2Sz4M3w).

It seems fair to me to remove the discussion about GHC Package DBs since I imagine everyone is using cabal 1.20.