Create an example of using the reader monad

I'm new to Haskell and don't yet grok monad transformers. I wanted to add some state to my Scotty application, so I looked for an example of using `scottyT`. The only one I found was [globalstate.hs](https://github.com/scotty-web/scotty/blob/3bd7a09feb906be6d549c3d2f276b40a30eba6af/examples/globalstate.hs), which complicated things by including STM. So I decided to pare that down and create my own example. 
I think a short comment at the top of this file that describes what this example illustrates would be useful.