docs: explain annotation naming

With check runs, we could specify the naming ourselves. With the switch
to workflow commands, it depends on how consumers of this action name
the consuming action.

README.md

@@ -15,6 +15,7 @@ name: Balto
 on: [pull_request]
 
 jobs:
+  # Note: the name of this job will be how annotations are labeled
   balto-eslint:
     runs-on: ubuntu-latest
     steps: