Document differences with tuareg

[bbatsov]

I've noticed this topic is generating a lot of confusion (see https://discuss.ocaml.org/t/tuareg-and-caml-modes-for-emacs-what-are-the-differences/10285). Both modes are very similar at a glance:

• major mode
• ocamldebug integration
• top-level integration

It'd be really nice if the README explained what are the differences between tuareg and caml-mode, so it's easier for people to decide which mode to use. I've also created a similar ticket for tuareg (ocaml/tuareg#298).

[Chris00]

I don't use caml-mode so cannot really compare. (IMHO, the two should be merged — I cannot do it due to lack of time.)