Added new getting started guide

Thanks @MinnDevelopment for his awesome work on
the JDA guide that had no influence here at all.

docs/guides/samples/intro/message.cs

@@ -0,0 +1,14 @@
+public async Task MainAsync()
+{
+	// client.Log ...
+	client.MessageReceived += MessageReceived;
+	// ...
+}
+
+private async Task MessageReceived(SocketMessage message)
+{
+	if (message.Content == "!ping")
+	{
+		await message.Channel.SendMessageAsync("Pong!");
+	}
+}