Add code with syntax highlighting on

GitHub already supports syntax highlighting in code samples in markdown files, so we can use it. That will get the code easier to read.

    ~~~purescript
    class Functor f where
        map :: forall a b. (a -> b) -> f a -> f b
    ~~~

Renders as:

~~~purescript
class Functor f where
    map :: forall a b. (a -> b) -> f a -> f b
~~~

That should make it more better in my opinion.