Provide a 'Changelog' link on rubygems.org/gems/csv (#327)

By providing a 'changelog_uri' in the metadata of the gemspec a
'Changelog' link will be shown on https://rubygems.org/gems/csv 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 <kou@cozmixng.org>

Author: mark-young-atg

csv.gemspec

@@ -55,4 +55,6 @@ Gem::Specification.new do |spec|
   spec.extra_rdoc_files = rdoc_files
 
   spec.required_ruby_version = ">= 2.5.0"
+
+  spec.metadata["changelog_uri"] = "#{spec.homepage}/releases/tag/v#{spec.version}"
 end