add readme with instructions to the _macro_tool (#107)

pkgs/_macro_tool/README.md

@@ -0,0 +1,15 @@
+A tool for running macro examples end to end.
+
+## Example usage
+
+The following will run the json_codable example, assuming you start at the root
+of this repo:
+
+```bash
+dart pkgs/_macro_tool/bin/main.dart \
+  --workspace=goldens/foo \
+  --packageConfig=.dart_tool/package_config.json \
+  --script=goldens/foo/lib/json_codable_test.dart
+```
+
+You can also enable watch mode using `--watch`.