Create Redux Best Practices + design goals Document

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.

Start with the Redux style guide at https://redux.js.org/style-guide/style-guide and review the contributor-docs repository structure to locate the best place for the document. Gather examples from the extension and mobile products, then outline guidance for new and existing stores, long-term Redux design goals, and relationships with MetaMask Core and the controllers repo. Done means the document is ready for ratification by both engineering groups.

Written by the indexing model from the issue text.