(#7506) Organize READMEs; specify supported Ruby versions in README.md

The README was not specific enough about the current state of our Ruby support.
This commit lays out our current status as discussed in issue #7506 and in
greater detail on the internal mailing list around April 8, 2011 under the
subject "Statler Ruby Versions Support."

This commit also consolidates two redundant README files into one.

Paired-With: Matt Robinson <matt@puppetlabs.com>