Indicate license in the Readme and documentation site

[greggish]

I just realized it's not easy to see what the license is on our docs site or in the README. I suppose there's some complexity in that there are different assets with different licenses (usually varying according to software vs conceptual products), though the primary license is CC BY-SA 4.0 and we aim to keep other licenses consistent with that.

[dan-odsc]

Closing #495