Add quickstart section to Readme

The existing Readme is missing your example how to get up and running, and get a feel for the library quickly. It would be great if this Readme could also be used as the landing page at https://hackage.haskell.org/package/opt-env-conf-0.5.0.1 @NorfairKing Many thanks for this awesome library. Feel free to modify, request changes or remove this PR.