Publish a style/design guide for algebra

[non]

It doesn't have to be very long, but I think it would be smart to explicitly document:

• goals (and non-goals) for the project, to be explicit about scope.
• design principles, to explain why we built it the way we did.
• expected usage patterns, to show how we imagine folks using this.
• expected extension patterns, to show how folks can fill in gaps or build on this.
• some basic code guidelines, to aid consistency
• high-level explanation about law-checking, and how we verify particular laws.

There is a little bit of this in Spire right now, but we could probably use more there, and since this project has a more limited scope it should be easier to do it here.

[johnynek]

+1 to this. The readme says some of goals, but not much about the rest.