Update quickstart.md

Textual tweaks that hopefully make it more newcomer friendly.
In Mac OSX, cabal installs binaries to: `~/Library/Haskell/bin/`.
In Windows Vista or newer Windows, cabal installs binaries to: `%APPDATA%\cabal\bin`.
`%APPDATA` is Windows specific Environment variable.
Usually, this variable specifies `C:\Users\<username>\AppData\Roaming`.