Add info, that current lib is not compatible with RxJava2 in Readme

[AlexTrotsenko]

I know, that there are issues like following:
#38
#33

Issue: after looking in the Readme it's not that clear, that it's not compatible with RxJava2 (which is mostly used now days)

Adding link to https://github.com/android10/frodo2 might be useful as well.