docs: Add main branch warning (#314)

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

Author: Lenny Goodell

README.md

@@ -1,6 +1,11 @@
 # edgexfoundry/device-snmp-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-snmp-go/releases).**
 
 #### Overview:
 A generic SNMP device service. Included is a sample for a network switch device driver. Additionally, a Patlite profile is included in the profiles director.