Update DefaultValuesObjectGraphVisitor.cs

add a description attribute about property.
if used,when Serialize(),the result will have a yaml comment on that property.

Author: 齐天大胖子

YamlDotNet/Serialization/ObjectGraphVisitors/DefaultValuesObjectGraphVisitor.cs

@@ -67,7 +67,15 @@ public override bool EnterMapping(IPropertyDescriptor key, IObjectDescriptor val
                     break;
             }
 
+            if (yamlMember != null)
+            {
+                if (yamlMember.Description != null)
+                {
+                    context.Emit(new Core.Events.Comment(yamlMember.Description, false));
+                }
+            }
+
             return base.EnterMapping(key, value, context);
         }
     }
-}
+}