Hint users how to let the ignore comments take effect (#475)

* Update README.md

* Rephrase the sentence a bit

---------

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

README.md

diff --git a/README.md b/README.md
index 45abf84..b6e947d 100644
--- a/README.md
+++ b/README.md
@@ -87,6 +87,8 @@
   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