Update README to reduce some confusion.

Like issue #142, I was confused by the text stating that ruby-install didn't need to be updated to get the latest version of Ruby. I followed the directions to get the latest version of 1.9, but didn't end up getting the latest version. This change adds some text to the README that will hopefully clarify things for the next person who runs into a similar situation.

Thanks,
Craig

Maybe "does not require.... out. (Knows a list of versions, but can be used to install versions not in its list.)"