Update MQTTnet.nuspec

Build/MQTTnet.nuspec

@@ -12,6 +12,7 @@
     <requireLicenseAcceptance>true</requireLicenseAcceptance>
     <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>
+* [Client] Increased delay for keep alive checks do decrease CPU load.
 * [Core] Decreased object allocations (#1324, thanks to @gfoidl).
 * [Client] Fixed issue in _MqttApplicationMessageBuilder.WithPayload_ (#1322, thanks to @gfoidl).