Make the (existing) licence clearer

Per `awesome-cv.cls`, this project is available under the Latex Project
Public Licence, v1.3c (SPDX: LPPL-1.3c) from the copyright holder Claud
D. Park <[email protected]> (aka @posquit0).

This commit adds a LICENCE file with a copy of that licence, to
hopefully make it clearer to users by showing up more conveniently in
GitHub for example.

Closes #20, closes #245, and closes #296.

Author: OJFord