Update the changelog.

Author: andersio

CHANGELOG.md

@@ -1,9 +1,12 @@
 # master
 
+1. When unfair locks from libplatform are unavailable, ReactiveSwift now fallbacks to error checking Pthread mutexes instead of the default. Mitigations regarding issues with `pthread_mutex_trylock` have also been applied. (#654, kudos to @andersio)
 
-# 4.0.0-rc.21. Support Swift 4.2 (Xcode 10) (#644, kudos to @ikesyo) 
+# 4.0.0-rc.2
 
-# 4.0.0-rc.1*Please add new entries at the top.*
+1. Support Swift 4.2 (Xcode 10) (#644, kudos to @ikesyo) 
+
+# 4.0.0-rc.1
 
 1. `Lifetime` may now be manually ended using `Lifetime.Token.dispose()`, in addition to the existing when-token-deinitializes semantic. (#641, kudos to @andersio) 
 1. For Swift 4.1 and above, `BindingSource` conformances are required to have `Error` parameterized as exactly `NoError`. As a result, `Signal` and `SignalProducer` are now conditionally `BindingSource`. (#590, kudos to @NachoSoto and @andersio)