gnu: Add ruby-yard.

* gnu/packages/ruby.scm (ruby-yard): New variable.

gnu/packages/ruby.scm

@@ -5536,3 +5536,24 @@ (define-public ruby-pdf-core
     (description "This is an experimental gem that extracts low-level PDF
 functionality from Prawn.")
     (license license:gpl3+)))
+
+(define-public ruby-yard
+  (package
+    (name "ruby-yard")
+    (version "0.9.16")
+    (source (origin
+              (method url-fetch)
+              (uri (rubygems-uri "yard" version))
+              (sha256
+               (base32
+                "0lmmr1839qgbb3zxfa7jf5mzy17yjl1yirwlgzdhws4452gqhn67"))))
+    (build-system ruby-build-system)
+    (arguments `(#:test-target "spec"))
+    (home-page "https://yardoc.org/")
+    (synopsis "Ruby documentation tool")
+    (description "YARD is a documentation generation tool for the Ruby
+programming language.  It enables the user to generate consistent, usable
+documentation that can be exported to a number of formats very easily, and
+also supports extending for custom Ruby constructs such as custom class level
+definitions.")
+    (license license:expat)))