add markdown badges

Author: devoncarew

README.md

@@ -1,3 +1,7 @@
+[![Dart CI](https://github.com/dart-lang/source_map_stack_trace/actions/workflows/test-package.yml/badge.svg)](https://github.com/dart-lang/source_map_stack_trace/actions/workflows/test-package.yml)
+[![pub package](https://img.shields.io/pub/v/source_map_stack_trace.svg)](https://pub.dev/packages/source_map_stack_trace)
+[![package publisher](https://img.shields.io/pub/publisher/source_map_stack_trace.svg)](https://pub.dev/packages/source_map_stack_trace/publisher)
+
 `source_map_stack_trace` is a package for converting stack traces generated by
 dart2js-compiled JavaScript code into readable native Dart stack traces using
 source maps. For example: