Update FUNDING.yml

add NumFOCUS github sponsors button (recurring donations only)

This feature is launching today at GitHub Universe!

Also suggested change to your donation link so that it's the one hosted on numfocus.org, in case that helps give potential donors a greater sense of security.

cc @story645 @tacaswell

Author: Dr-G

.github/FUNDING.yml

@@ -1,3 +1,3 @@
 # These are supported funding model platforms
-
-custom: https://numfocus.salsalabs.org/donate-to-matplotlib/index.html
+github: [numfocus]
+custom: https://numfocus.org/donate-to-matplotlib