Type at least n characters before you can submit the form

This is not a duplicate of this question since that one is about searching.

We have a simple form containing one text area and a submit button.

^{download bmml source – Wireframes created with Balsamiq Mockups}

The user has to type at least 100 characters before she can submit the message. How can we inform the user about this restriction?

Sure, we can inform the user by placing the restriction below the field name. But, since users tend not to read text, we have decided do disable the button until at least 100 characters have been typed. The user still needs to know about this limitation.

How can we solve this in a user-friendly way?