docs: Add repo main branch warning (#232)

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

README.md

@@ -1,6 +1,10 @@
 # Official EdgeX Foundry Examples
 [![Go Report Card](https://goreportcard.com/badge/github.com/edgexfoundry/edgex-examples)](https://goreportcard.com/report/github.com/edgexfoundry/edgex-examples) [![GitHub Pull Requests](https://img.shields.io/github/issues-pr-raw/edgexfoundry/edgex-examples)](https://github.com/edgexfoundry/edgex-examples/pulls) [![GitHub Contributors](https://img.shields.io/github/contributors/edgexfoundry/edgex-examples)](https://github.com/edgexfoundry/edgex-examples/contributors) [![GitHub Committers](https://img.shields.io/badge/team-committers-green)](https://github.com/orgs/edgexfoundry/teams/edgex-examples-committers/members) [![GitHub Commit Activity](https://img.shields.io/github/commit-activity/m/edgexfoundry/edgex-examples)](https://github.com/edgexfoundry/edgex-examples/commits)
 
+> **Warning**  
+> The **main** branch of this repository contains work-in-progress examples for the upcoming release, and is **not guaranteed to be stable or work with previous releases**.
+>
+> **Examples that work with the latest EdgeX release can be found at [Releases](https://github.com/edgexfoundry/edgex-examples/releases).** Navigate to the latest (x.y) tag.
 
 The purpose of this repository is to provide centralized example and sample code for EdgeX adopters and users.  The folders help organize the examples to make it easier to find samples associated to various parts of EdgeX.