Version correspondence

[linpopopo]

Version correspondence between RxBinding and RxJava?

[JakeWharton]

You can infer this from the change log, but I agree it's not the best documentation.

Basically:

• RxBinding 1.x (written in Java) --> RxJava 1.x
• RxBinding 2.x (written in Java) --> RxJava 2.x
• RxBinding 3.x (written in Kotlin) --> RxJava 2.x
• RxBinding 4.x (written in Kotlin) --> RxJava 3.x

If you want to send a PR adding this information somewhere in the README I would accept it.