Add FUNDING.yml

Display a "Sponsor" button at the top of the repository.
More information:
https://github.blog/2019-05-23-announcing-github-sponsors-a-new-way-to-contribute-to-open-source/
https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository

.github/FUNDING.yml

@@ -0,0 +1,2 @@
+liberapay: SimonBrazell
+custom: https://www.buymeacoffee.com/SimonBrazell