Add warning about git tags (#359)

* Add warning about git tags

* Correct emoji

README.md

@@ -1,5 +1,7 @@
 # OpenTelemetry Collector distributions
 
+> :warning: **Important note:** Git tags in this repository may change at any time to fix any issues found during a release. They are only meant to trigger Github releases and should not be relied upon.
+
 This repository assembles OpenTelemetry Collector distributions, such as the "core" distribution, or "contrib". It may contain non-official distributions, focused on specific use-cases, such as the load-balancer.
 
 Each distribution contains: