add explicit LICENSE file

[jennybc]

FYI there's already license info in the footer of all rendered pages but I should do this w/ explicit LICENSE file

[jennybc]

apparently it's not even in the footer of index

[daattali]

It looks like the "this work is licensed under ..." will by default appear on all pages, but it's deliberately removed from several pages such as the homepage. Was there any reason to purposely not include that footer on some pages? If not, that can be easily fixed

[jennybc]

I think it was an aesthetic choice? That I justifed in my mind because there really isn't any content to license on such a page? I don't feel strongly about this. I just want it to be clear people can reuse the stuff.