docs: [Browser] Define the support browser runtimes #4168

README.md

@@ -135,8 +135,17 @@ Note that versions of Node.JS v8 prior to `v8.12.0` will NOT work, because OpenT
 > Client instrumentation for the browser is **experimental** and mostly **unspecified**. If you are interested in
 > helping out, get in touch with the [Client Instrumentation SIG][client-instrumentation-sig].
 
-There is currently no list of officially supported browsers. OpenTelemetry is developed using standard web
-technologies and aims to work in currently supported versions of major browsers.
+Rather that define versions of specific browsers / runtimes, OpenTelemetry sets the minimum supported version based on the
+underlying Language features used.
+
+The current minumum language feature support is set as [ECMAScript 2020](https://www.w3schools.com/jS/js_2020.asp) that are available
+in all modern browsers / runtimes.
+
+This means that if you are targeting or your end-users are using a browser / runtime that does not support ES2020, you will need
+to either transpile your code to ensure compatibility or provide any necessary polyfill's for the missing features. Any support
+issues that arise from using a browser or runtime that does not support ES2020 will be closed as "won't fix".
+
+This minimum support level is subject to change as the project evolves and as the underlying language features evolve.
 
 ## Package Version Compatibility