lookahead documentation

This is a really great project, I'm having a blast learning about parser combinators, thanks so much for your work. 

I noticed there was no documentation about the lookahead parser so I tried to add something that will hopefully be useful to other newbies like me. Please take a look and if you think it'd be helpful please feel free to pull it in. 

But also, if this isn't the intended use for lookahead or if you can think of a better example, or if my documentation is confusing in anyway, just let me know and feel free to reject. 

Thanks again, 
Dave