Document hiding Dashboards in a How To section (thoughtbot#1781)

Begins the changes for thoughtbot#1778

Closes thoughtbot#1587

Co-authored-by: Tino <tino@klassapp.com>

docs/how_to.md

@@ -0,0 +1,21 @@
+---
+title: How To
+---
+
+## Hiding Dashboards from the Sidebar
+
+Resource can be removed form the sidebar by removing their index action from the routes. For example:
+
+```ruby
+# config/routes.rb
+Rails.application.routes.draw do
+  namespace :admin do
+    resources :line_items, except: :index
+    resources :orders
+    resources :products
+    root to: "customers#index"
+  end
+end
+```
+
+In this case, only Orders and Products will appear in the sidebar, while Line Items can still appear as an association.