Add guidelines for maintaining changelogs

Changelogs seem straightforward enough to maintain, but recent updates
in the `core` repo indicate that there may be misconceptions. The
guidelines in this PR aim to clarify the purpose of a changelog and how
to maintain them so that they are valuable for consumers at all times.

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

README.md

@@ -6,6 +6,7 @@ This is a living repository — nothing is set in stone! If you're member of Met
 
 ## Table of Contents
 
+- [Changelog Guidelines](./docs/changelogs.md)
 - [Contributor Code of Conduct](https://github.com/MetaMask/.github/blob/main/CODE_OF_CONDUCT.md)
 - [Engineering Principles](./docs/engineering-principles.md)
 - [JavaScript Guidelines](./docs/javascript.md)