Add project metadata to the gemspec (brianmario#1089)

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 and
source_code_uri links will appear on the rubygems page at
https://rubygems.org/gems/mysql2 and be available via the rubygems API
after the next release.

Author: orien

mysql2.gemspec

@@ -10,6 +10,13 @@ Mysql2::GEMSPEC = Gem::Specification.new do |s|
   s.homepage = 'https://github.com/brianmario/mysql2'
   s.rdoc_options = ["--charset=UTF-8"]
   s.summary = 'A simple, fast Mysql library for Ruby, binding to libmysql'
+  s.metadata = {
+    'bug_tracker_uri'   => "#{s.homepage}/issues",
+    'changelog_uri'     => "#{s.homepage}/releases/tag/#{s.version}",
+    'documentation_uri' => "https://www.rubydoc.info/gems/mysql2/#{s.version}",
+    'homepage_uri'      => s.homepage,
+    'source_code_uri'   => "#{s.homepage}/tree/#{s.version}",
+  }
   s.required_ruby_version = '>= 2.0.0'
 
   s.files = `git ls-files README.md CHANGELOG.md LICENSE ext lib support`.split