Updated documentation for output handling

Hi, based on issue #115, this is my sugested update to the documentation regarding the output handling. I tried the Blackhole and Abstract formatters but couldn't get them to work so I haven't mentioned them. Dot format did work (in that it didn't throw an error) but I didn't actually see any dots! However, that may just be due to what I was testing and it may well be fine for others so I have said a few words about it. Hope that helps.