readme: better document what happens when remote_url fails

Maybe the last pull for a while dealing with remote_urls? One can only hope.

The same research that led to #1011 suggests the delightfully-automatic invalid URL handling under ActiveRecord should be documented. 

(Related: I've also updated the [known issues](https://github.com/jnicklas/carrierwave/wiki/Known-Issues) with details on why issues like #1003 are currently unavoidable.)