Provide a 'Changelog' link on rubygems.org/gems/stringio (#111)

By providing a 'changelog_uri' in the metadata of the gemspec a
'Changelog' link will be shown on https://rubygems.org/gems/stringio
which makes it quick and easy for someone to check on the changes
introduced with a new version.

Details of this functionality can be found on
https://guides.rubygems.org/specification-reference/#metadata

---------

Co-authored-by: Sutou Kouhei <[email protected]>

Author: mark-young-atg

stringio.gemspec

@@ -39,6 +39,8 @@ Gem::Specification.new do |s|
   s.required_ruby_version = ">= 2.7"
   s.summary = "Pseudo IO on String"
 
+  s.metadata["changelog_uri"] = "#{s.homepage}/releases/tag/v#{s.version}"
+
   # s.cert_chain  = %w[certs/nobu.pem]
   # s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/
 end