Add guidelines for maintaining changelogs

Changelogs seem straightforward enough to maintain, but we've noticed
some misconceptions around them that cause them to be less useful than
they should be in various projects where they are used. These guidelines
explain what changelogs are good for and how to maintain them in such a
way that they stay good.

This is a prequisite for updating the release process in the `core`
monorepo but should prove valuable in many other MetaMask projects as
well.

Author: mcmire