Clarify RxJava 3+ support (#66)

Since [RxJava have decided to not accept the BlockHound integration](ReactiveX/RxJava#6692),
we should clarify what users of the future versions of RxJava should do
to enable BlockHound with RxJava.

If RxJava community decide to add an integration with BlockHound,
we will update it accordingly.

README.md

@@ -64,7 +64,8 @@ Although BlockHound supports [the SPI mechanism to integrate with](https://githu
 1. [Project Reactor](https://projectreactor.io)  
   Version 3.2.x is supported out of the box.  
   Starting with `reactor-core` version 3.3.0, there is [a built-in integration in Reactor itself](https://github.com/reactor/reactor-core/blob/v3.3.0.RELEASE/reactor-core/src/main/java/reactor/core/scheduler/ReactorBlockHoundIntegration.java) that uses [the SPI](https://github.com/reactor/BlockHound/blob/master/docs/custom_integrations.md).
-2. [RxJava 2](https://github.com/ReactiveX/RxJava/)
+2. [RxJava 2](https://github.com/ReactiveX/RxJava/) is supported.  
+    RxJava 3 and further versions of RxJava will require an SPI to be implemented, either by the framework or user. See [this PR to RxJava](https://github.com/ReactiveX/RxJava/pull/6692) with an example of the SPI's implementation.
 
 # Quick Start
 See [the docs](./docs/README.md).