Add project metadata to the gemspec (#172)

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/public_suffix and be available via the
rubygems API after the next release.

public_suffix.gemspec

@@ -12,6 +12,14 @@ Gem::Specification.new do |s|
   s.description = "PublicSuffix can parse and decompose a domain name into top level domain, domain and subdomains."
   s.licenses    = ["MIT"]
 
+  s.metadata = {
+    "bug_tracker_uri" => "https://github.com/weppos/publicsuffix-ruby/issues",
+    "changelog_uri" => "https://github.com/weppos/publicsuffix-ruby/blob/master/CHANGELOG.md",
+    "documentation_uri" => "https://rubydoc.info/gems/#{s.name}/#{s.version}",
+    "homepage_uri" => s.homepage,
+    "source_code_uri" => "https://github.com/weppos/publicsuffix-ruby/tree/v#{s.version}",
+  }
+
   s.required_ruby_version = ">= 2.3"
 
   s.require_paths    = ["lib"]