add failing Chronic.parse example to README
add failing Chronic.parse example to README

Chronic.parse silently returns nil when invalid input is given.
This might confuse some people.

To avoid possible confusions in the future, this commit
explicitly mentions this behavior in README.

davispuh/chronic,lautis/chronic,CraigCottingham/chronic,j-hernandez/chronic,copiousfreetime/chronic,mojombo/chronic