Add an "approach to data" document

[nylen]

This should be similar to the corresponding document for Calypso but not as complicated and not yet as fully-formed.

It should explain a bit about what Redux is and how it works within React applications, as well as how network requests fit into this structure (triggered by Redux actions using refx and using the WP REST API Backbone client library).

[youknowriad]

related #902

[aduth]

related #902

Maybe even duplicate of?

[nylen]

Related, but I think doing this and documenting it are separate tasks. #902 also relates more to network actions, but our documentation should cover the structure of the application at a more basic level.

[mtias]

Going to close this as we'd cover things by documenting the tools (withApiData, etc).