Implement REUSE

[vitormattos]

Just the concerns of an external contributor, but it's not a good idea to commit third-party logos to the repository without explicitly clarifying the license.

As your website is AGPL licensed, by including these logos, it looks like you're attempting redistribute these assets under AGPL as well, which obviously isn't right.

If you're certain you're allowed to use these images in this way, I'd recommend using a tool like REUSE by the FSFE for declaring the licenses properly.

This way you're explicit about the license or copyright holder of each file.

Originally posted by @SethFalco in #247 (comment)

to-do

• Add the follow action:
  https://github.com/LibreSign/libresign/blob/main/.github/workflows/reuse.yml
• Check if the follow tool will work fine
  • Directory LICENSES exists
  • All licenses used stay inside this folder
  • All not binary and not json files at this repository have a SPDX header
  • File REUSE.toml was created
    https://github.com/LibreCodeCoop/spdx-header-updater
• Check if the action will work fine without errors

[lfals]

Implemented at the open PR #357