Add documentation link into gemspec

Author: AlexWayfer

uri.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
+  spec.metadata["documentation_uri"] = "https://rubydoc.info/gems/uri"
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.