Update dependency description for RxJava

[senpl]

Implementation of RxJava3 require change from Rx2Timer to Rx3Timer, change imports and in dependencies com.github.thepacific:rx3-timer io.reactivex.rxjava3:rxjava version up to 3.0.6

Implementation of RxJava2 requires com.github.thepacific:rx2-timer io.reactivex.rxjava2:rxjava

This should be in readme or somewhere as without this dependencies errors appears.