Update doco to automatically generate coverage

[pappes]

The instructions tell the user to manually run
flutter test --coverage

The better solution is for the user to change user settings
Shift-Ctrl-P Preferences: Open User Settings (JSON)

and as the last setting add
"dart.flutterTestAdditionalArgs": ["--coverage"]
this will mean every time the developer runs a test the code coverage gutters will get updated with the latest coverage information