Add project metadata to the gemspec

As per https://guides.rubygems.org/specification-reference/#metadata,
add metadata to the gemspec file. This'll allow people to more easily
access the source code, raise issues and read the changelog. These
bug_tracker_uri, changelog_uri, documentation_uri, homepage_uri
source_code_uri, and wiki_uri links will appear on the rubygems page at
https://rubygems.org/gems/listen and be available via the rubygems API
after the next release.

listen.gemspec

@@ -16,7 +16,13 @@ Gem::Specification.new do |gem|
   gem.description = 'The Listen gem listens to file modifications and '\
     'notifies you about the changes. Works everywhere!'
   gem.metadata = {
-    'allowed_push_host' => 'https://rubygems.org'
+    'allowed_push_host' => 'https://rubygems.org',
+    'bug_tracker_uri' => "#{gem.homepage}/issues",
+    'changelog_uri' => "#{gem.homepage}/releases",
+    'documentation_uri' => "https://www.rubydoc.info/gems/listen/#{gem.version}",
+    'homepage_uri' => gem.homepage,
+    'source_code_uri' => "#{gem.homepage}/tree/v#{gem.version}",
+    'wiki_uri' => "#{gem.homepage}/wiki"
   }
 
   gem.files = `git ls-files -z`.split("\x0").select do |f|