A guide to new users on how to use UniMath

[nmvdw]

This suggestion is inspired by a discussion on the UniMath Zulip.

It can be confusing for newcomers to use UniMath, because there are some differences between UniMath and plain Coq (i.e., Set, Prop, exists). It would be good to add a such a guide for newcomers and link to it in the README file.