2.0 Design: ReactiveX Design Guidelines and Contract

[benjchristensen]

The Rx Design Guidelines posted by Microsoft is out of date and does not correctly represent RxJava. We need to update or rewrite it to match the evolution over the past couple years, such as the addition of backpressure.

Related: #981 and ReactiveX/reactivex.github.io#14

cc @headinthebox @mattpodwysocki @jhusain as we would preferably do this across RxJS, Rx.Net, etc. However, if we can't reach consensus, RxJava will stick closer to Reactive Streams and j.u.c.Flow which is targeting Java 9 and based on Reactive Streams. This is because interop within the JVM ecosystem is more important than between languages. This is similar to how ES7 is evolving an "Observable" that has the necessary semantics but differs significantly from .Net and Java.

[benjchristensen]

This is required to achieve #2450