Proposal: ContT : add minimal usage example of delimited continuations

[sjshuck]

[original issue 80 by @ocramz]

Since shift/reset are not intuitive, I'd like to add at least one example to the ContT haddock, something along the lines of :

-- | demonstration of non-local control flow with a single shift/reset pair
--
-- λ> run t1
-- ('b',"b_a")
t1 :: ContT Char (State [Char]) Char
t1 = resetT $ do
  let
    x = 'a' -- input
    cons w = lift $ modify (w :)
  r <- shiftT $ \k -> do
    cons x -- initial state uses the input
    let x&#39; = succ x -- compute a function of the input
    y <- lift $ k x&#39; -- delegate to the continuation k
    cons y -- mutate state with the return value of k
    pure x&#39;
  cons &#39;_&#39;
  pure r

[sjshuck]

This is a completely fine issue.