Add guidelines around when to use peerDependencies vs. dependencies

[mcmire]

While maintaining Wallet Framework packages, we have found that controller dependencies and type-only dependencies frequently get miscategorized. It would be good to have a resource we can point people to whenever confusion arises.

Acceptance Criteria

• A document exists which explains:
  • Why peerDependencies exists and what it does
  • Situations in which filing a dependency under devDependencies + peerDependencies is desired over dependencies (e.g. controllers)
  • Situations in which it is not appropriate to file a dependency under devDependencies + peerDependencies (e.g. type-only dependencies)
  • When and how to bump peerDependencies in a monorepo

[mcmire]

Also see: MetaMask/core#5336