Added DocFX documentation

docs/concepts/CustomMessages.md

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+# Creating custom NMEA messages
+
+Custom NMEA messages can be registered for parsing as well.
+To create a new message, add the NmeaMessageType attribute to the class, and declare the 5-character message type.
+
+*Note: You can use `--` as the first two characters to make it independent of the Talker Type.*
+
+Next ensure you have a constructor that takes the `TypeName` string parameter first, and a second `string[]` parameter that will contain all the message values:
+
+
+Example:
+```cs
+    [NmeaMessageType("PTEST")]
+    public class CustomMessage : NmeaMessage
+    {
+        public CustomMessage(string type, string[] parameters) : base(type, parameters)
+        {
+            Value = parameters[0];
+        }
+        public string Value { get; }
+    }
+```
+
+Next register this with the NMEA Parser using either:
+
+```cs
+NmeaMessage.RegisterAssembly(typeof(CustomMessage).Assembly); //Registers all types in the provided assembly
+NmeaMessage.RegisterMessage(typeof(CustomMessage).GetTypeInfo());  //Registers a single NMEA message
+```
+
+Note that these methods will throw if the NMEA type has already been registers (there's an overload where you can declare the `replace` parameter to `true` to overwrite already registered messages.
+
+Next you should be able to test this method using the Parse method:
+```cs
+var input = "$PTEST,TEST*7C";
+var msg = NmeaMessage.Parse(input);
+```
+
+# Creating a multi-sentence message
+
+A NMEA message cannot exceed 82 characters, so often messages are split into multiple sentences. To create a custom multi message, either implement `IMultiSentenceMessage` or simply subclass `NmeaMultiSentenceMessage`.
+
+
+```cs
+[NmeaMessageType("PTST2")]
+private class CustomMultiMessage : NmeaMultiSentenceMessage, IMultiSentenceMessage
+{   
+    public CustomMultiMessage(string type, string[] parameters) : base(type, parameters)
+    {
+    }
+    public string Id { get; private set; }
+    public List<string> Values { get; } = new List<string>();
+	// Set index in the message where the total count is:
+    protected override int MessageCountIndex => 0;
+	// Set index in the message where the message number is:
+    protected override int MessageNumberIndex => 1;
+    protected override bool ParseSentences(Talker talkerType, string[] message)
+    {
+	    // Ensure this message matches the previous message.
+		// Use any indicator to detect message difference, so you can to error out and avoid
+		// appending the wrong message
+        if (Id == null)
+            Id = message[2]; //First time it's not set
+        else if (Id != message[2])
+            return false;
+        Values.AddRange(message.Skip(3));
+        return true;
+    }
+}
+```
+Note that the message is parsed in the `ParseSentences` method, and not the constructor. Also note that the talkerType is parsed to you, because multi-sentence messages allows a mix of talker types, if you use the `--` prefex in the NMEA type.
+
+Next we can parse the two messages and have the second one be appended to the first one:
+
+```cs
+NmeaMessage.RegisterNmeaMessage(typeof(CustomMultiMessage).GetTypeInfo());
+var input1 = "$PTST2,2,1,123,A,B,C,D*2A";
+var input2 = "$PTST2,2,2,123,E,F,G,H*21";
+var msg1 = NmeaMessage.Parse(input1);
+var msg2 = NmeaMessage.Parse(input2, msg1 as IMultiSentenceMessage);
+```
+
+If msg1 and msg2 aren't the same instance, it means the message couldn't be added to the previous message, and a new message was generated.