mqtt: Add KNOWN_ISSUES file

The KNOWN_ISSUES file describes the following:

- "Unable to connect" and timeout errors
- Publisher and Subscriber limitations

Jira: ZEP-428
Jira: ZEP-632
Jira: ZEP-669

Change-Id: I10591a1fd14bccbeb0b7f3876549d29ddf8ffd1f
Signed-off-by: Flavio Santes <flavio.santes@intel.com>

samples/net/paho_mqtt_clients/publisher/KNOWN_ISSUES

@@ -0,0 +1,30 @@
+Issues and Limitations
+----------------------
+
+- Unable to connect
+	Caused by:
+		Incorrect TCP flags handling from the Zephyr IP Stack.
+	Workaround:
+		Reset the board.
+
+- Publish error
+- Subscribe error
+        Caused by:
+		The MQTT broker is sending another MQTT message while the
+		client is still processing the QoS handshake from a
+		previous MQTT message. MQTT QoS in Zephyr is implemented
+		in a synchronous way. For asynchronous implementations, see:
+		http://git.eclipse.org/c/mosquitto/org.eclipse.mosquitto.git/tree/
+	Workaround 1:
+		Reduce the speed/amount of messages received by the
+		application.
+	Workaroung 2:
+		Use the mqtt_read function to handle all the incoming
+		messages.
+
+- RX or TX error
+	Caused by:
+		Perhaps the rx/tx timeouts are too short for the LAN.
+        Workaround:
+		Increase the rx or tx timeout.
+		See: publisher/src/netz.c, struct netz_ctx_t.

samples/net/paho_mqtt_clients/subscriber/KNOWN_ISSUES

@@ -0,0 +1 @@
+See ../publisher/KNOWN_ISSUES