add donation links to GitHub FUNDING.yml
https://github.blog/2019-05-23-announcing-github-sponsors-a-new-way-to-contribute-to-open-source/ fdroiddata!5041 fdroidserver!668
https://github.blog/2019-05-23-announcing-github-sponsors-a-new-way-to-contribute-to-open-source/ fdroiddata!5041 fdroidserver!668