Update gemspec with URLs

Otherwise it's hard to figure out where to contribute to the gem.

asciimath.gemspec

@@ -11,9 +11,16 @@ Gem::Specification.new do |spec|
   spec.email         = ["[email protected]", "[email protected]"]
   spec.summary       = %q{AsciiMath parser and converter}
   spec.description   = %q{A pure Ruby AsciiMath parsing and conversion library.}
-  spec.homepage      = ""
+  spec.homepage      = "https://asciidoctor.org/"
   spec.license       = "MIT"
 
+  spec.metadata = {
+    "bug_tracker_uri" => "https://github.com/asciidoctor/asciimath/issues",
+    "changelog_uri" => "https://github.com/asciidoctor/asciimath/blob/HEAD/CHANGELOG.adoc",
+    "mailing_list_uri" => "https://chat.asciidoctor.org",
+    "source_code_uri" => "https://github.com/asciidoctor/asciimath"
+  }
+
   spec.files         = `git ls-files -z`.split("\x0")
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})