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