Add todo about model validation

src/Core/Models/Model.php

@@ -14,6 +14,12 @@ use LogicException;
  * Adds the ability for custom relations to be added to a model during runtime.
  * These relations behave in the same way that you would expect; they can be
  * queried, eager loaded, and accessed as an attribute.
+ *
+ * @todo Refactor out validation, either into a trait or into a dependency.
+ *       The following requirements need to be fulfilled:
+ *       - Ability for extensions to alter ruleset.
+ *       - Ability for extensions to add custom rules to the validator instance.
+ *       - Use Flarum's translator with the validator instance.
  */
 abstract class Model extends Eloquent
 {