The example of extending Ember.TextField with required is not relevant anymore

Since the `required` attribute is part of the recognised attributes as listed on line 27, the example about extending Ember.TextField shouldn't be about extending with `required`. 

However I am under the impression that default "coming soon" behaviour is to accept all attributes by default instead of specifying them. If that's the case the whole Extension paragraph might be irrelevant.
Merge pull request #4729 from rlopes/master

The example of extending Ember.TextField with required is not relevant anymore