Merge pull request #14 from PolymerElements/fix-docs

add docs for this thing

Author: notwaldorf

iron-validatable-behavior.html

@@ -14,7 +14,20 @@
 <script>
 
   /**
-   * Use `Polymer.IronValidatableBehavior` to implement an element that validates user input.
+   * `Use Polymer.IronValidatableBehavior` to implement an element that validates user input.
+   * Use the related `Polymer.IronValidatorBehavior` to add custom validation logic to an iron-input.
+   *
+   * By default, an `<iron-form>` element validates its fields when the user presses the submit button.
+   * To validate a form imperatively, call the form's `validate()` method, which in turn will
+   * call `validate()` on all its children. By using `Polymer.IronValidatableBehavior`, your
+   * custom element will get a public `validate()`, which
+   * will return the validity of the element, and a corresponding `invalid` attribute,
+   * which can be used for styling.
+   *
+   * To implement the custom validation logic of your element, you must override
+   * the protected `_getValidity()` method of this behaviour, rather than `validate()`.
+   * See [this](https://github.com/PolymerElements/iron-form/blob/master/demo/simple-element.html)
+   * for an example.
    *
    * ### Accessibility
    *