Update README for current release.

The current README recommends installing an RC which is not compatible with the latest published version found on rubygems which I personally find counter-intuitive and confusing.
I think we should at least mention this explicitly and link to the docs for the current release.
I also think there are quite a few people who will want to use 1.3.2 in any case (me for example) since ohm-contrib doesnt work with the current RC.