Add Sponsor button to GitHub

These past months I've spent a lot of time working on xmonad and I feel
like I've done a lot. This is, however, not sustainable long term. :-(

I'd like to try making my GitHub Sponsors profile a bit more visible,
hoping that would allow me to continue dedicating time to xmonad.

I know that the correct approach probably is for the xmonad project to
find a fiscal sponsor like the Software Freedom Conservancy or Open
Collective or something and accept donations as a project, and then
redistribute that to people, but I don't think the project has enough
momentum to do something as complicated as that. :-/

.github/FUNDING.yml

@@ -0,0 +1,2 @@
+github:
+  - liskin