Rental 0.4.15 Released

Version 0.4.15 of rental is published, adding the README to and a minor convenience method.

A new method called try_new_or_drop has been added to rental structs. This version differs from try_new in that the head value passed in is dropped on failure, allowing the method to return a normal Result containing only the error value. This allows the method to participate more smoothly with existing error conversions without needing to use map_err to manually discard the head field.