Add CC-BY-SA-4.0 license (#126)

This applies to the new single-page content. It's the same license I use for the book.

Future contributions should be licensed under these terms. See https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license

> Whenever you make a contribution to a repository containing notice of a license, you license your contribution under the same terms, and you agree that you have the right to license your contribution under those terms. If you have a separate agreement to license your contributions under different terms, such as a contributor license agreement, that agreement will supersede.

> Isn't this just how it works already? Yep. This is widely accepted as the norm in the open-source community; it's commonly referred to by the shorthand "inbound=outbound". We're just making it explicit.

Author: paf31