Add Open Collective and Liberapay to FUNDING.yml

Source: https://www.borgbackup.org/support/fund.html

.github/FUNDING.yml

@@ -1,5 +1,6 @@
 # These are supported funding model platforms
 
 # github: # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
-# liberapay: borgbackup
+liberapay: borgbackup
 custom: ['https://www.borgbackup.org/support/fund.html']
+open_collective: borgbackup