Add changelog_uri to gemspec (#851)

Supported here:
https://guides.rubygems.org/specification-reference/#metadata Useful for
running https://github.com/MaximeD/gem_updater

overcommit.gemspec

@@ -15,6 +15,10 @@ Gem::Specification.new do |s|
   s.post_install_message  =
     'Install hooks by running `overcommit --install` in your Git repository'
 
+  s.metadata = {
+    'changelog_uri' => 'https://github.com/sds/overcommit/blob/main/CHANGELOG.md'
+  }
+
   s.require_paths         = %w[lib]
 
   s.executables           = ['overcommit']