updated releasenotes

Build/MQTTnet.nuspec

@@ -20,6 +20,7 @@
 * [Server] Added support for assigned client IDs (MQTTv5 only) (thanks to @bcrosnier).
 * [Server] Added interceptor for unsubscriptions.
 * [MQTTnet.Server] Added interceptor for unsubscriptions.
+* [MQTTnet.AspNetCore] improved compatibility with AspNetCore 3.1
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2019</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags>