Add Buy Me a Coffee sponsorship link

Author: Sanjaya-Danushka

.github/FUNDING.yml

@@ -10,4 +10,4 @@ github: [Sanjaya-Danushka]
 # liberapay: # Replace with a single Liberapay username
 # issuehunt: # Replace with a single IssueHunt username
 # otechie: # Replace with a single Otechie username
-# custom: # Replace with up to 4 custom sponsorship URLs e.g., ['https://www.example.com/sponsor1', 'https://www.example.com/sponsor2']
+custom: ['https://buymeacoffee.com/sanjayadanushka']