Update docs.

Build/MQTTnet.nuspec

@@ -13,6 +13,7 @@
     <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] Added support for TLS 1.3 (requires .NET Core 3.1+) (thanks to @Dvergatal).
+* [Core] Added support for _MaximumQoS_ transmit when using MQTTv5 (thanks to @nayato).
 * [RpcClient] Moved the RPC topic validation to the topic generation strategy.
 * [RcpClient] Added interface for _MqttRpcClient_.
 * [Server] Fixed bug in PubRel packet generation (MQTTv5 only).