updating readme with link to @ryanb's solution

Author: John Reilly

README.markdown

@@ -49,4 +49,8 @@ The `test/unit/part_assignment_test.rb` file has several simple tests that demon
 Run `rake test` to run the tests.
 
 
-Any ideas?
+Any ideas?
+
+# Solutions
+
+* [ryanb](https://github.com/ryanb) suggests moving the validation into PartAssignment instead of Part. See [this branch](https://github.com/johnreilly/polymorphic_validation_scope/tree/ryanb).