Split usage samples in README into sections.

[artem-zinnatullin]

Helps with readability and also allows users to make a link to particular section!

[arturdryomov]

Maybe fix the remaining styling issues?

• Get value or fall back to something else if None (getOrElse())
• Filter only None values emitted by RxJava 2 or Project Reactor
• Check if Optional is Some or None