Liberapay is now officially recognised, update FUNDING.yml (#5386)

.github/FUNDING.yml

@@ -1,3 +1,4 @@
 # One username per supported platform and one custom link
 patreon: matrixdotorg
-custom: https://liberapay.com/matrixdotorg
+liberapay: matrixdotorg
+custom: https://paypal.me/matrixdotorg

changelog.d/5386.misc

@@ -0,0 +1 @@
+Add a sponsor button to the repo.