License terms for docs

[johnstv]

It doesn't look like there's any mention of the license for the documentation articles anymore.

There previously was a LICENSE.md file at the root of this repo a few months ago that mentioned the docs were licensed under the "Creative Commons Attribution-ShareAlike 4.0 International License" but it seems that it was removed. The following is a link to the LICENSE.md file as it was on 12 June 2021 before it was removed: https://github.com/system76/docs/blob/91fdefbdcf63396263b285af6fbffc4006b0c7ba/LICENSE.md

Would it be possible to add a License file to the repo to give clarity about the terms?