Finish PR 77 - Add guide explaning impredicativity

Description of the change

Finishes what was started and nearly completed in #77. Credit goes to @i-am-tom.

Checklist:

[x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
[ ] Linked any existing issues or proposals that this pull request should close
[ ] Updated or added relevant documentation in the README and/or documentation directory
[ ] Added a test for the contribution (if applicable)

purescript-contrib / purescript-profunctor-lenses