Document RxKotlin2 (RxKotlin version of RxJava 2.x)

[DavidMGross]

As with RxJava, the documentation of operators & such for RxKotlin will split into 1.x and 2.x versions.

[DavidMGross]

See ReactiveX/RxKotlin#87

[akarnokd]

RxKotlin is currently abandoned so probably can'd do much about it here.