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

jaro_winkler.gemspec

@@ -18,6 +18,12 @@ Gem::Specification.new do |spec|
   UTF-8, EUC-JP, Big5, etc.'
   spec.homepage = 'https://github.com/tonytonyjan/jaro_winkler'
   spec.license = 'MIT'
+  spec.metadata = {
+    'bug_tracker_uri' => 'https://github.com/tonytonyjan/jaro_winkler/issues',
+    'changelog_uri' => "https://github.com/tonytonyjan/jaro_winkler/blob/v#{spec.version}/CHANGELOG.md",
+    'documentation_uri' => "https://www.rubydoc.info/gems/jaro_winkler/#{spec.version}",
+    'source_code_uri' => "https://github.com/tonytonyjan/jaro_winkler/tree/v#{spec.version}",
+  }
   spec.files = Dir['lib/**/*.rb', 'ext/**/*.{h,c}', 'LICENSE.txt']
   spec.add_development_dependency 'bundler', '~> 1.7'
   spec.add_development_dependency 'rake', '~> 12.0'

jaro_winkler.java.gemspec

@@ -17,6 +17,12 @@ Gem::Specification.new do |spec|
   UTF-8, EUC-JP, Big5, etc.'
   spec.homepage = 'https://github.com/tonytonyjan/jaro_winkler'
   spec.license = 'MIT'
+  spec.metadata = {
+    'bug_tracker_uri' => 'https://github.com/tonytonyjan/jaro_winkler/issues',
+    'changelog_uri' => "https://github.com/tonytonyjan/jaro_winkler/blob/v#{spec.version}/CHANGELOG.md",
+    'documentation_uri' => "https://www.rubydoc.info/gems/jaro_winkler/#{spec.version}",
+    'source_code_uri' => "https://github.com/tonytonyjan/jaro_winkler/tree/v#{spec.version}",
+  }
   spec.files = Dir['lib/**/*.rb', 'LICENSE.txt']
   spec.add_development_dependency 'bundler', '~> 1.7'
   spec.add_development_dependency 'rake', '~> 12.0'