Update release notes

Build/MQTTnet.nuspec

@@ -11,7 +11,9 @@
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>MQTTnet is a .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
     <releaseNotes>* [.NET Standard] Added support for .NET Standard 1.3 (Thanks to 1iveowl)
-* [Server] Added an even which allows processing of all received messages from the clients</releaseNotes>
+* [Server] Added an even which allows processing of all received messages from the clients
+* [Server] Fixed an issue when stopping the server
+    </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2017</copyright>
     <tags>MQTT MQTTClient MQTTServer MQTTBroker Broker</tags> 
     <dependencies>