Create Redux Best Practices + design goals Document

[Brad Decker]

Note, https://redux.js.org/style-guide/style-guide is essentially our best practices document. However it might serve to use this document as a starting point, reference it and encapsulate some examples from either product to show what it means. This document will help to guide engineers in creating new stores in Redux, as well as in modifying exisitng ones. For the goals portion of this document we should describe what some long term goals would be for the redux store, some of the design considerations and how they will work together with things like MetaMask Core, The controllers repo, etc. This document will be ratified by members of both @MetaMask/extension-devs and @MetaMask/mobile-devs and will be continuously updated as our needs change.