Add short "Who is this for" section at the start of both haddock docs and README.md

This should give readers a quick idea of who is this for and what are the benefits.

Mention this library is for those that are dealing a lot with different filepaths and want to make sure they are correctly concatenating and passing them around, that no paths are getting mixed up, be it regarding to their standard (posix/win), their relativity (rel vs abs) or their nature (file vs dir).