Add 2.1 -> 2.2 upgrade guide

Finally.

guides/content/developer/upgrades/index.md

@@ -16,3 +16,4 @@ If there are any issues with these guides, please let us know by [filing an issu
 * [1.2.x to 1.3.x](/developer/upgrades/one-dot-two-to-one-dot-three)
 * [1.3.x to 2.0.x](/developer/upgrades/one-dot-three-to-two-dot-oh)
 * [2.0.x to 2.1.x](/developer/upgrades/two-dot-oh-to-two-dot-one)
+* [2.1.x to 2.2.x](/developer/upgrades/two-dot-one-to-two-dot-two)

guides/content/developer/upgrades/two-dot-one-to-two-dot-two.md

@@ -0,0 +1,59 @@
+---
+title: Upgrading Spree from 2.1.x to 2.2.x
+section: upgrades
+---
+
+This guide covers upgrading a 2.1.x Spree store, to a 2.2.x store. This
+guide has been written from the perspective of a blank Spree 2.1.x store with
+no extensions.
+
+If you have extensions that your store depends on, you will need to manually
+verify that each of those extensions work within your 2.2.x store once this
+upgrade is complete. Typically, extensions that are compatible with this
+version of Spree will have a 2-2-stable branch.
+
+## Upgrade Rails
+
+For this Spree release, you will need to upgrade your Rails version to at least 4.0.6.
+
+```ruby
+gem 'rails', '~> 4.0.6'
+```
+
+## Upgrade Spree
+
+For best results, use the 2-2-stable branch from GitHub:
+
+```ruby
+gem 'spree', :github => 'spree/spree', :branch => '2-2-stable'```
+
+Run `bundle update spree`.
+
+## Copy and run migrations
+
+Copy over the migrations from Spree (and any other engine) and run them using
+these commands:
+
+ rake railties:install:migrations
+ rake db:migrate
+
+## Read the release notes
+
+For information about changes contained with this release, please read the [2.2.0 Release Notes](http:https://guides.spreecommerce.com/release_notes/spree_2_2_0.html).
+
+### Rename assets
+
+As mentioned in the release notes, asset paths have changed. Change the references on the left, to the ones on the right:
+
+* `admin/spree_backend` => `spree/backend`
+* `store/spree_frontend` => `spree/frontend`
+
+This applies across the board on Spree, and may need to be done in your store's extensions.
+
+### Paperclip settings have been removed from master
+
+Please consult [this section](http:https://guides.spreecommerce.com/release_notes/spree_2_2_0.html#paperclip-settings-have-been-removed) of the release notes if you were using custom Paperclip settings. This will direct you what to do in that particular case.
+
+## Verify that everything is OK
+
+Click around in your store and make sure it's performing as normal. Fix any deprecation warnings you see.