Add CHANGELOG

[andreasabel]

ghc-paths has a number of releases, 0.1, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.1.0.4, 0.1.0.5, 0.1.0.6, 0.1.0.7, 0.1.0.8, 0.1.0.9, 0.1.0.12,
but misses a CHANGELOG that would explain the rationales for the different releases. (E.g. it is not clear to me whether I need to bound the version of ghc-paths from below in my projects.)

Please add a CHANGELOG.