docs: Added a parenthetical comment about ngModel being required

[wesleycho]

This PR is to make absolutely clear to users that ngModel is required in order to take advantage of FormController from the form directive for publishing inputs onto $scope.

[mary-poppins]

Thanks for the PR!

• Contributor signed CLA now or in the past
  • If you just signed, leave a comment here with your real name
• PR's commit messages follow the commit message format

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!

[petebacondarwin]

Landed with modifications. Nice idea thanks.