Add funding URI metadata to the gemspec

slideck.gemspec

@@ -16,6 +16,7 @@ Gem::Specification.new do |spec|
   spec.metadata["bug_tracker_uri"] = "https://github.com/piotrmurach/slideck/issues"
   spec.metadata["changelog_uri"] = "https://github.com/piotrmurach/slideck/blob/master/CHANGELOG.md"
   spec.metadata["documentation_uri"] = "https://www.rubydoc.info/gems/slideck"
+  spec.metadata["funding_uri"] = "https://github.com/sponsors/piotrmurach"
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["rubygems_mfa_required"] = "true"
   spec.metadata["source_code_uri"] = "https://github.com/piotrmurach/slideck"