Add clearer docs around coroutine support with an example (#5799)

extensions/kotlin/README.md

@@ -0,0 +1,12 @@
+# OpenTelemetry Kotlin Extension
+
+Kotlin [Extensions](src/main/kotlin/io/opentelemetry/extension/kotlin/ContextExtensions.kt) to propagate
+OpenTelemetry context into coroutines.
+
+For example, you could do the following with coroutines
+
+```kotlin
+launch(Context.current().asContextElement()) {
+// trace ids propagated here
+}
+```