Update docs.

Build/MQTTnet.nuspec

@@ -12,6 +12,7 @@
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports v3.1.0, v3.1.1 and v5.0.0 of the MQTT protocol.</description>
     <releaseNotes>
 * [Core] Nuget packages with symbols are now also published to improve debugging.
+* [ManagedClient] Fix a race condition in the message storage (thanks to @PaulFake).
 * [Server] Added items dictionary to client session in order to share data across interceptors as along as the session exists.
 * [Server] Exposed CONNECT packet properties in Application Message and Subscription interceptor.
 * [Server] Fixed: Sending Large packets with AspnetCore based connection throws System.ArgumentException.