Add more documentation.

[original issue 30 by rprecenth]

As someone who fought a lot with monad transformers before I got them, I would like the documentation to include some more examples showing some use cases for the different transformers. I found some in Control.Monad.Trans.Class but that's not really a place someone would go to if they want to know about, say ExceptT.

I would like to give this a shot, but before I start I thought it would be best to check that there isn't already an ongoing project of this.