docs: Add main branch warning (#612)

Signed-off-by: Leonard Goodell <[email protected]>

Author: Lenny Goodell

README.md

@@ -1,6 +1,12 @@
 # Device MQTT Go
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+> **Warning**  
+> The **main** branch of this repository contains work-in-progress development code for the upcoming release, and is **not guaranteed to be stable or working**.
+> It is only compatible with the [main branch of edgex-compose](https://github.com/edgexfoundry/edgex-compose) which uses the Docker images built from the **main** branch of this repo and other repos.
+>
+> **The source for the latest release can be found at [Releases](https://github.com/edgexfoundry/device-mqtt-go/releases).**
+
 ## Overview
 MQTT Micro Service - device service for connecting a MQTT topic to EdgeX acting like a device/sensor feed.