Update README.md

Perhaps decide_format could be omitted as an API because it is more of an internal function in the same way as output_flask. I added all of comments together for @flask_rdf to consolidate them. I was a little unclear on add_format. It seems to indicate that you would just register a new two text string pair, but you mention plugins as a content negotiation system. I couldn't really see how it was plugged in, so I figured I was missing something. I hope the comment changes make sense :P