Add CHANGES.md and README.md to extra-source-files
Add CHANGES.md and README.md to extra-source-files

Doing so makes Hackage link directly to the changelog, and renders the README in the Hackage description of the library.
ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell