Add a barebones "design guide" with some content in a trade-offs chapter.

[nathan-at-least]

Check out the SUMMARY.md outline for some suggested sections. The trade-offs chapter has two specific topics that came up recently in Arborist.

Is this the right home for this scope of document? I want a home that's shared & managed across Zcash dev community, and the scope feels broader / looser than spec or ZIPs.