Add rubygems.org metadata (#349)

Author: m-nakamura145

config.gemspec

@@ -20,6 +20,11 @@ Please consider donating to our open collective to help us maintain this project
 \n
 Donate: \e[34mhttps://opencollective.com/rubyconfig/donate\e[0m\n"
 
+  s.metadata = {
+    'changelog_uri' => "https://github.com/rubyconfig/config/blob/master/CHANGELOG.md",
+    'source_code_uri' => 'https://github.com/rubyconfig/config',
+    'bug_tracker_uri' => 'https://github.com/rubyconfig/config/issues'
+  }
   s.files = `git ls-files`.split($/)
   s.files.select! { |file| /(^lib\/|^\w+.md$|\.gemspec$)/ =~ file }