Hint users how to let the ignore comments take effect (dart-archive/c…

…overage#475)

* Update README.md

* Rephrase the sentence a bit

---------

Co-authored-by: Liam Appelbe <liama@google.com>

pkgs/coverage/README.md

@@ -87,6 +87,8 @@ coverage will also be output.
   lines inclusive.
 - `// coverage:ignore-file` to ignore the whole file.
 
+Then pass `--check-ignore` to `format_coverage`.
+
 #### Function and branch coverage
 
 To gather function level coverage information, pass `--function-coverage` to