Update README.md with Android min version badge (#831)

Author: bidetofevil

README.md

@@ -3,6 +3,7 @@
 [![Continuous Build][ci-image]][ci-url]
 [![Maven Central][maven-image]][maven-url]
 [![OpenSSF Scorecard](https://api.scorecard.dev/projects/github.com/open-telemetry/opentelemetry-android/badge)](https://scorecard.dev/viewer/?uri=github.com/open-telemetry/opentelemetry-android)
+[![android api](https://img.shields.io/badge/Android_API-21-green.svg "Android min API 21")](VERSIONING.md)
 
 ## Status: Experimental