add .github/FUNDING.yml

This should add a :heart: Sponsor button for the repository.
https://help.github.com/en/articles/displaying-a-sponsor-button-in-your-repository

I thought this is so small change that I could do it directly instead of
opening an issue.

.github/FUNDING.yml

@@ -0,0 +1 @@
+custom: https://www.buymeacoffee.com/sduduzog