Added a new page to the documentation for customizing model attributes

This new page walks through an example of adding a custom attribute,
and contains the following sections:
- Adding attribute columns to the database, with a link to official
Rails migration documentation.
- Adding attribute fields to the admin panel, by using Deface,
with a link to and mention of the full Customizing Admin Panel page.
- Allowing custom attributes to be saved, by editing an initializer
and rebooting the Rails server.

guides/data/nav/developers.yml

@@ -52,6 +52,8 @@
         href: "/developers/customizations/customizing-assets.html"
       - title: "Customizing Permissions"
         href: "/developers/customizations/customizing-permissions.html"
+      - title: "Customizing Model Attributes"
+        href: "/developers/customizations/customizing-attributes.html"
   - title: "Extensions"
     dropdown:
       - title: "Installing extensions"

guides/source/developers/customizations/customizing-attributes.html.md

@@ -0,0 +1,92 @@
+# Customizing Model Attributes
+
+This guide covers how to add editable attributes to models in Solidus.
+We will cover an example of adding a new attribute to the `Spree:Taxon` model 
+to allow for an additional text area.
+
+-   Adding attribute columns to the database
+-   Adding attribute fields to the admin panel
+-   Allowing custom attributes to be saved
+
+## Adding Columns to the Database
+
+Columns can be added to Solidus database tables using standard Rails migrations
+[standard Rails migrations][standard_rails_migrations].
+
+If you wanted to add a `more_information` attribute, you could run the 
+following command:
+
+```bash
+rails generate migration AddMoreInformationToTaxons
+``` 
+
+This would create a migration file starting with a timestamp 
+`db/migrate/20190925192026_add_more_information_to_taxons.rb`. Then update the 
+file with the following contents to add a new text type column 
+`more_information` to the table:
+
+```ruby
+class AddMoreInformationToTaxons < ActiveRecord::Migration[5.2]
+  def change
+    add_column :spree_taxons, :more_information, :text
+  end
+end
+```
+
+After you are done editing your migration run the following command to add the 
+column to the database:
+
+```bash
+rails db:migrate
+```
+
+[standard_rails_migrations]: https://guides.rubyonrails.org/active_record_migrations.html
+
+## Adding Fields to the Admin Panel
+
+We have a complete guide on [customizing the admin panel][admin], but will
+go over it here briefly for our example. While that guide covers template
+replacement and using the [Deface][deface] gem, we'll just cover the 
+Deface approach. This allows changing parts of the views without overriding
+them entirely.
+
+To add a new form field for a custom attribute to the taxon form, we would
+want to extend `app/views/spree/admin/taxons/_form.html.erb` from the 
+Solidus backend. We can do that with Deface by creating a file 
+`app/overrides/spree/admin/taxons/_form/add_fields.html.erb.deface`.
+
+Below is an example of adding a new field to this form for a `more_information`
+field.
+
+```erb
+<!-- insert_bottom '[data-hook=admin_inside_taxon_form]' -->
+<div class="col-10">
+  <%= f.field_container :more_information do %>
+    <%= f.label :more_information %><br />
+    <%= f.text_area :more_information, class: 'fullwidth', rows: 6 %>
+  <% end %>
+</div>
+```
+
+[admin]: customizing-admin.html
+[deface]: https://github.com/spree/deface
+
+## Allowing Custom Attributes to be Saved
+
+If you try to submit the form with the new attribute field, you will notice
+it will not save to the database. This is due to every model having a list of
+permitted attributes.
+
+To extend this list, we can add the following line to an initializer, for
+example in `config/initializers/spree.rb`:
+
+```ruby
+Spree::PermittedAttributes.taxon_attributes << [:more_information]
+```
+
+This tells Solidus that it's okay to accept values being saved to your custom
+attributes. If you were going to do this for multiple attributes, you can
+comma separate them.
+
+After this has been added, you will need to reboot your Rails server. Now
+you can use the form in the admin panel to manage your custom attribute.

guides/source/developers/customizations/overview.html.md

@@ -16,9 +16,11 @@ and the framework that power Solidus, respectively.
 - [Learn How to Customize the Solidus Classes][decorators]
 - [Learn How to Customize the Assets][assets]
 - [Learn How to Customize the Permissions][permissions]
+- [Learn How to Customize Model Attributes][attributes]
 
 [storefront]: customizing-storefront.html
 [admin]: customizing-admin.html
 [assets]: customizing-assets.html
 [decorators]: decorators.html
 [permissions]: customizing-permissions.html
+[attributes]: customizing-attributes.html