Readonly input property would be useful if made available

Disabling the input field and not showing the date picker makes sense, but the use case for allowing the readonly property is it allows for focus on the input, allows for submission of the value, but prevents the user from typing their own custom event value, and therefore conforming to chosen moment format.
Merge pull request #681 from mtpultz/patch-1

Readonly input property would be useful if made available