Textures and models licenses specification

[mattiaverga]

As I'm packaging Celestia for Fedora Linux / EPEL, I'm particularly concerned by texture and 3d models packaged in celestia-data. Fedora requires that all content licenses must be FOSS and clearly specified, but I'm really struggling to track all different sources used here... for example, the recent addition of Vesta textures only reports the source site where they were downloaded from, but from a quick look at the website I can't find out how they're licensed.

I'd really like for future additions that a full license review is made before accepting any texture or model. Also, REUSE specification provides a good way to list what license is used for each file in a project. I'd love to see the quite obscure description transferred from the README file into the REUSE specification.