Add CC BY-SA 4.0

[machitgarha]

#28 does not really look good to me, because the files are not actually markdown, but just plain text. So I'm adding this, with the style built based on the original license in the Creative Commons website.

P.S.: It seems this repository is inactive, but someone might benefit from PRs as well. I hope it gets merged soon, however. :)

[millette]

See also https://github.com/idleberg/Creative-Commons-Markdown - got to catch'em all!