Rewrite README in RDoc format.

`README` has been rewritten in the RDoc format. The wording in several sections
was massaged to be more understandable, but no major changes were made. Same
deal with code examples.

As an aside: the documentation and code samples in some sections are *really*
bad. For example, the "attachment_fu migrations" section is barely
comprehensible.
Rename README -> README.rdoc

This has been done for two reasons:

* The file should render properly on GitHub w/a file extension.
* Adding a filename extension gives semantic info about the format of it's
  contents.

References to README were also corrected.