Add README explaining how this works and when it does not work

[andreasabel]

You can shoot yourself in the foot with this package.
It pretends to give you a valid location for GHC.
However, it only delivers the location of GHC when it itself was built.

There should be a README explaining how this package works, when to use and when not to use it.

See:

• https://stackoverflow.com/questions/38096298/where-is-the-source-of-ghc-paths-value-came-from
• GHC.Paths makes incorrect assumption as to where to find ghc #4