Add changelog URI to gemspec metadata (#1641)

# Description

Add changelog URI to gemspec metadata
# Completed Tasks

- [x] I have read the [Contributing Guide][contrib].
- [x] The pull request is complete (implemented / written).
- [x] Git commits have been cleaned up (squash WIP / revert commits).
- [x] I wrote tests and ran `bundle exec rake` locally (if code is
attached to PR).

[contrib]: https://github.com/lsegal/yard/blob/main/CONTRIBUTING.md

Author: lsegal

yard.gemspec

@@ -20,4 +20,5 @@ Gem::Specification.new do |s|
   s.executables   = ['yard', 'yardoc', 'yri']
   s.license = 'MIT' if s.respond_to?(:license=)
   s.metadata['yard.run'] = 'yri'
+  s.metadata['changelog_uri'] = 'https://rubydoc.info/gems/yard/file/CHANGELOG.md'
 end