Define return_value and return_type in XcodegenAction

Author: liamnichols

lib/fastlane/plugin/xcodegen/actions/xcodegen_action.rb

@@ -40,6 +40,14 @@ def self.details
         ].join("\n")
       end
 
+      def self.return_type
+        :string
+      end
+
+      def self.return_value
+        "Raw shell output of `xcodegen` command."
+      end
+
       def self.available_options
         [
           FastlaneCore::ConfigItem.new(key: :executable,