Add note about build_stubbed and Marshal.dump

[ci skip]

Closes #1120

GETTING_STARTED.md

@@ -187,6 +187,9 @@ user.first_name
 # => "Joe"
 ```
 
+Note that objects created with `build_stubbed` cannot be serialized with
+`Marshal.dump`, since factory_bot defines singleton methods on these objects.
+
 Static Attributes
 ------------------