document the range or results

[raould]

(hi, thanks for this library - very excited to be able to seed things, so e.g. if i try to make a game i can experiment with replaying things by reusing a seed.)

it would be great for a newbie coming to this library for the return values to be documented e.g. i /assume/ that double() returns something in the range 0...1, because that's often how other prng's work, but it isn't explicit that i could see in e.g. the readme. people like me who are easily confused would get a leg up with such clarifications. :-)