Upgrade Provider - Create Design Doc

[SBGoods]

Maintenance of the standard library providers prioritises stability and correctness relative to the provider's intended feature set. Create a design document describing this feature set, and any other design considerations which influence the architecture of the provider and what can and cannot be added to it.

As part of the utility provider upgrade guidance we need to create a DESIGN.md document. An example of this document can be found in the TLS provider

Relates to: #50

[davidcorrigan714]

This is desperately needed. It's very clear from trying to use this provider that the use cases for it have not all been fully tested by the dev team. The examples are pretty useless, showing no real world or advanced use cases. Anyone trying to use the rfc3339 or rotation_rfc3339 arguments is bound to hit issues, I couldn't find a single working example for either of them.