Add README.

Author: msqr

README.md

@@ -0,0 +1,33 @@
+# MQTT.fx CBOR payload decoder
+
+This project contains an addon for the [MQTT.fx][mqttfx] application that can decode CBOR MQTT
+message payloads so they are displayed as formatted JSON.
+
+![MQTT.fx screenshot of CBOR decoded message](docs/mqttfx-cbor-decoded-message.png)
+
+
+# Install
+
+You can download a precompiled JAR from the [releases][releases] page. The
+**mqtt-cbor-decoder-X-all.jar** file should be placed in the MQTT.fx **addon** directory, which
+varies by platform:
+
+| OS          | Add-on location                                          |
+|:------------|:---------------------------------------------------------|
+| **macOS**   | `[USER_HOME]/Library/Application Support/MQTT-FX/addons` |
+| **Windows** | `[USER_HOME]\AppData\Local\MQTT-FX\addons`               |
+| **Linux**   | `[USER_HOME]/MQTT-FX/addons`                             |
+
+
+# Building
+
+The build uses Gradle. Build with the `shadowJar` task:
+
+```
+./gradlew shadowJar
+```
+
+That will produce a `build/libs/mqttfx-cbor-decoder-X-all.jar` which you can install.
+
+[mqttfx]: http://mqttfx.org/
+[releases]: https://github.com/SolarNetwork/mqttfx-cbor-decoder/releases