Merge pull request #1055 from chkr1011/add-code-doc

Added client documentation.

Source/MQTTnet/MqttTopicFilterBuilder.cs

@@ -11,7 +11,21 @@ namespace MQTTnet
 
     public class MqttTopicFilterBuilder
     {
+        /// <summary>
+        /// The quality of service level.
+        /// The Quality of Service (QoS) level is an agreement between the sender of a message and the receiver of a message that defines the guarantee of delivery for a specific message.
+        /// There are 3 QoS levels in MQTT:
+        /// - At most once  (0): Message gets delivered no time, once or multiple times.
+        /// - At least once (1): Message gets delivered at least once (one time or more often).
+        /// - Exactly once  (2): Message gets delivered exactly once (It's ensured that the message only comes once).
+        /// </summary>
         MqttQualityOfServiceLevel _qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce;
+
+        /// <summary>
+        /// The MQTT topic.
+        /// In MQTT, the word topic refers to an UTF-8 string that the broker uses to filter messages for each connected client.
+        /// The topic consists of one or more topic levels. Each topic level is separated by a forward slash (topic level separator).
+        /// </summary>
         string _topic;
         bool? _noLocal;
         bool? _retainAsPublished;