Reformat README.

The README has been reformatted in the following ways:

1. Bare code samples have been wrapped in + or <code> tags as appropriate.
2. The large code sample in <h2>has_attachment(options = {})</h2> has been
   re-ordered so examples are sorted alphabetically.
3. The following transformations were applied to several code samples:

        foo :bar -> foo(:bar)
        { foo: :bar } -> {foo: :bar}

4. Other minor tweaks. Typo corrections and the like.