Created MQTT support (markdown)

MQTT-support.md

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+The payload is formatted in JSON notation. The measurement reference is the Unix date/time in milliseconds.
+
+Currently only the weight of an user is transmitted. You could test the openScale sync MQTT support for example using [CloudMQTT](https://www.cloudmqtt.com/).
+
+_Note: openScale supported only MQTT 3.1 **not** MQTT 5.x_
+
+**Adding a measurement**
+
+topic `openScaleSync/measurements/insert`
+
+payload `{"date":1572082380000,"weight":80.0}`
+
+**Update a measurement**
+
+topic `openScaleSync/measurements/update`
+
+payload `{"date":1572082380000,"weight":120.0}`
+
+**Delete a measurement**
+
+topic `openScaleSync/measurements/delete`
+
+payload `{"date":1572082380000}`
+
+**Clear all measurements**
+
+topic `openScaleSync/measurements/clear`
+
+payload `true`