show console message mentioning --run-codemods

Now that codemods are opt-in, if they run without --run-codemods, we should print to console telling them about the option. Mentioned in https://github.com/kellyselden/ember-cli-update/pull/217#pullrequestreview-81219929.

Prerequisite is #109

cc @rwjblue

Start by reviewing prerequisite issue #109 and the codemod execution path in this repository. Reproduce a run where codemods execute without --run-codemods, then identify the existing console-output location; done means the command clearly tells users about the option without changing opt-in behavior.

Written by the indexing model from the issue text.