add example of how to use list pool

[justinmeiners]

This partially addresses #39.

• It adds an example of how to use the list, and informs the reader to refer to it through an exercise.
• add footnote about pop_back
• add example of how to use iterators

[aharrison24]

That's really good, thanks.

I find it frustrating that the allocate member function doesn't compose as well as cons does in lisp. I couldn't help myself but try and fix that (in a verbose C++98 style to maintain Stepanovianism!)
https://godbolt.org/z/Efj5G8bdK

Perhaps you could set that as an exercise! 😂 (not serious!)