Add content license for non-code portions

[chris m]

Per the Rust Embedded Book, it looks like it's common to include a license for the prose sections of a book as well. I believe that they use the CC-BY-SA 4.0 license, terms of which can be seen here.

[Lorenz Schmidt]

the CC license reads reasonable, the ShareAlike section reads a bit like a GPL license for prose, which I think is the way to go here. On my part we can double-license the code under MIT/Apache and the prose as CC-BY-SA 4.0

[chris m]

That sounds like a plan to me. I think in the long-term I'd like to keep an eye on some of the ethical license stuff I've mentioned in the past regarding code, but since most of that stuff is still somewhat under construction, I'm pretty happy with staying with MIT/Apache2.0 at the moment as well. CC-BY-SA 4.0 for the prose sounds fine to me either way. I'll plan to push those licenses in the next day or two

[chris m]

Resolved per commit 35f6125