Add Rails 5 usage note to README

README.md

@@ -22,12 +22,27 @@ This gem requires Ruby 2.3.1 as of version 4.3. If you need to use an older Ruby
 
 ### Installation
 
-To easily install or upgrade to the latest release, use [gem](http://rubygems.org/)
+Add `shopify_api` to your `Gemfile`:
+
+```ruby
+gem 'shopify_api'
+```
+
+Or install via [gem](http://rubygems.org/)
 
 ```bash
 gem install shopify_api
 ```
 
+#### Rails 5
+
+shopify_api is compatible with Rails 5 but since the latest ActiveResource release (4.1) is locked on Rails 4.x, you'll need to use the unreleased master version:
+
+```ruby
+gem 'shopify_api'
+gem 'activeresource', github: 'rails/activeresource'
+```
+
 ### Getting Started
 
 ShopifyAPI uses ActiveResource to communicate with the REST web service. ActiveResource has to be configured with a fully authorized URL of a particular store first. To obtain that URL you can follow these steps: