Document parsing in the readme files

[atmoos]

Parsing functions a bit differently to conventional types

Possibly expected

Length twoMetres = Length.Parse("2 m"); // this doesn't exist!

This library

var unitRepository = UnitRepository.Create(); // can load other assemblies containing custom units
IParser<Length> parser = Parser.Create<Length>(unitRepository); // use that to create a parser

Length twoMetres = parser.Parse("2 m");