Add note about BlockHound (Kotlin#2465)

README.md

@@ -45,6 +45,7 @@ suspend fun main() = coroutineScope {
 * [debug](kotlinx-coroutines-debug/README.md) — debug utilities for coroutines:
  * [DebugProbes] API to probe, keep track of, print and dump active coroutines;
  * [CoroutinesTimeout] test rule to automatically dump coroutines on test timeout.
+ * Automatic integration with [BlockHound](https://github.com/reactor/BlockHound).
 * [reactive](reactive/README.md) — modules that provide builders and iteration support for various reactive streams libraries:
  * Reactive Streams ([Publisher.collect], [Publisher.awaitSingle], [kotlinx.coroutines.reactive.publish], etc), 
  * Flow (JDK 9) (the same interface as for Reactive Streams),