Add documentation for Using custom attributes in menus and taxonomy archives (#40755)

docs/extension-development/using-custom-attributes-in-menus.md

@@ -0,0 +1,38 @@
+
+# Using custom attributes in menus and taxonomy archives
+
+Attributes that can be used for the layered nav are a custom taxonomy, which means you can display them in menus, or display products by attributes. This requires some work on your part, and archives must be enabled.
+
+> **Note:**  This is a  **Developer level**  doc. If you are unfamiliar with code/templates and resolving potential conflicts, select a  [WooExpert or Developer](https://woocommerce.com/customizations/) for assistance. We are unable to provide support for customizations under our  [Support Policy](http://www.woocommerce.com/support-policy/).
+
+# Register the taxonomy for menus
+
+When registering taxonomies for your custom attributes, WooCommerce calls the following hook:
+
+```php
+$show_in_nav_menus = apply_filters('woocommerce_attribute_show_in_nav_menus', false, $name);
+```
+
+So, for example, if your attribute slug was `size` you would do the following to register it for menus:
+
+```php
+add_filter('woocommerce_attribute_show_in_nav_menus', 'wc_reg_for_menus', 1, 2);
+
+function wc_reg_for_menus( $register, $name = '' ) {
+if ( $name == 'pa_size' ) $register = true;
+return $register;
+}
+```
+
+Custom attribute slugs are prefixed with `pa_`, so an attribute called `size` would be `pa_size`
+
+Now use your attribute in  **Appearance > Menus**. You will notice, however, that it has default blog styling when you click on a link to your taxonomy term.
+
+# Create a template
+
+You need to theme your attribute to make it display products as you want. To do this:
+
+1.  Copy `woocommerce/templates/taxonomy-product_cat.php` into your theme folder
+2.  Rename the template to reflect your attribute – in our example we’d use `taxonomy-pa_size.php`
+
+You should now see this template when viewing taxonomy terms for your custom attribute.