Add documentation on selectors

[mcmire]

Creating selector functions to access parts of controller state is not something that we have been promoting actively. This is mainly due to the fact that we don't have documentation on this pattern. However, it's something we all teams should be aware of so that we can reduce the duplication between clients. We should add documentation on this.

This ticket depends on https://github.com/MetaMask/decisions/pull/32.