Document "types" module in README

[NeQuissimus]

Just stumbled upon eu.timepit.refined.types.all. Let's add something about this package to the README :)

[fthomas]

Agreed! Most of the README was written before the types package was added. In the early days of the library, the focus was on the predicates while it currently shifts more towards fully refined types. Nowadays I'm using type aliases like PosInt more than explicit types like Int Refined Positive.