Fix #44 by stating which libraries are supported OOTB in README (#60)

- add Reactor's built-in integration mention to README
- add RxJava's built-in integration mention to README

README.md

@@ -59,6 +59,13 @@ Where:
 |`$LATEST_MILESTONE`|[![](https://img.shields.io/badge/dynamic/xml.svg?label=&color=blue&query=%2F%2Fmetadata%2Fversioning%2Flatest&url=https%3A%2F%2Frepo.spring.io%2Fmilestone%2Fio%2Fprojectreactor%2Ftools%2Fblockhound%2Fmaven-metadata.xml)](https://repo.spring.io/milestone/io/projectreactor/tools/blockhound/)|
 |`$LATEST_SNAPSHOT`|[![](https://img.shields.io/badge/dynamic/xml.svg?label=&color=orange&query=%2F%2Fmetadata%2Fversioning%2Flatest&url=https%3A%2F%2Frepo.spring.io%2Fsnapshot%2Fio%2Fprojectreactor%2Ftools%2Fblockhound%2Fmaven-metadata.xml)](https://repo.spring.io/snapshot/io/projectreactor/tools/blockhound/)|
 
+## Built-in integrations
+Although BlockHound supports [the SPI mechanism to integrate with](https://github.com/reactor/BlockHound/blob/master/docs/custom_integrations.md), it comes with a few built-in integrations:
+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/)
+
 # Quick Start
 See [the docs](./docs/README.md).