Provide a 'Changelog' link on rubygems.org/gems/bigdecimal (#281)

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

bigdecimal.gemspec

@@ -52,4 +52,6 @@ Gem::Specification.new do |s|
   end
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
+
+  s.metadata["changelog_uri"] = s.homepage + "/blob/master/CHANGES.md"
 end