Reactor module readme update (#2260)

Author: meiao

instrumentation/reactor-3.3.0/README.md

@@ -1,10 +1,19 @@
 # Reactor Instrumentation
 
-Instrumentation for Reactor Core library code.
+Instrumentation for Reactor Core library code. This module provides mostly the same functionality of `netty-reactor-0.9.0`, but this will only apply when netty
+is not being used.
 
 This instrumentation module is a subset of the `netty-reactor-0.9.0` instrumentation. It does not contain anything related to HTTP nor starting transactions and has added Skips for when `reactor-netty` classes are present.
 
-The contents of the `netty-reactor` module were not moved to this module because it would cause the `tokenLift` to register twice in the `Hooks` class.  
+Changes to `netty-reactor` should be mirrored here and vice-versa.
+
+## Why not separate the functionality?
+`netty-reactor` modules register the `tokenLift` in the Hooks class from two different code paths, one from reactor and another from netty. To make sure that
+`tokenLift` is registered only once, an AtomicBoolean new field is added to the `Hooks` class. Before registering, that field is checked and if false,
+`tokenLift` is registered and the field is set to true.
+
+The code cannot be separated in modules because the new field is not visible across modules, so there would be no way to make sure that `tokenLift` gets
+registered only once.
 
 ## Notice
 This module will only properly link the code if the Mono/Flux is subscribed on a scheduler.