[FIXUP][TODO][ARCHITECTURE.md] Add ActiveRecord section

Seems like this would be a good section to have to allow new developers
to understand how we are processing requests for additional attributes
in the API and what the flow through helper classes is for that.

[skip ci]

Author: NickLaMuro

ARCHITECTURE.md

@@ -27,6 +27,10 @@ Table of Contents
       - [Validate Action](#validate-action-validate_action)
       - [Custom Actions](#custom-actions-custom_actions)
   - [Drawing Routes (the `config/routes.rb`)](#drawing-routes-the-configroutesrb)
+* [Working with ActiveRecord](#working-with-activerecord)
+  - [How custom result sets are built][#how-custom-result-sets-are-built]
+  - [Attributes and VirtualAttributes][#attributes-and-virtualattributes]
+  - [Rbac][#rbac]
 * [The `manageiq-api-client` gem](#the-manageiq-api-client-gem)
 
 
@@ -195,6 +199,16 @@ actions for a specific resource in the DB without giving it a direct route to
 call.  Custom buttons attached to VMs are an example of this in practice.
 
 
+Working with ActiveRecord
+-------------------------
+
+### How custom result sets are built
+
+### Attributes and VirtualAttributes
+
+### Rbac
+
+
 The `manageiq-api-client` gem
 -----------------------------