Provide a 'Changelog' link on rubygems.org/gems/terser

By providing a 'changelog_uri' in the metadata of the gemspec a
'Changelog' link will be shown on https://rubygems.org/gems/terser
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/

terser.gemspec

@@ -35,4 +35,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.0"
   spec.add_development_dependency "sourcemap", "~> 0.1.1"
+
+  spec.metadata["changelog_uri"] = spec.homepage + "/blob/master/CHANGELOG.md"
 end