add FUNDING.yml for GitHub Sponsors publicity
GitHub has a new beta program called "Sponsors" which is a donation service. It is a closed beta, so you have to apply to join. I applied:
Yes, this is GitHub, not GitLab. This is relevant since the main repos have been mirrored on GitHub for years: https://github.com/f-droid/ So might as well have some donation publicity there. So I added the Liberapay, GitHub, and other links. If any other contributor signs up for the GitHub Sponsors beta, then please add yourself to the list in this file.
They also support quite a few other donation services like Patreon and OpenCollective which might be worth trying out. Here are the options: https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository
since its about money I'm pinging @all