add additional install steps to README

Thanks to @gerep in #spree for pointing out the omission.

README.md

@@ -10,9 +10,17 @@ Behind-the-scenes, this extension uses [PayPal's Merchant Ruby SDK](https://gith
 
 ## Installation
 
-Add this extension to your Gemfile with this line:
+1. Add this extension to your Gemfile with this line:
 
-    gem 'spree_paypal_express', :github => "radar/better_spree_paypal_express", :branch => "2-0-stable"
+        gem 'spree_paypal_express', :github => "radar/better_spree_paypal_express", :branch => "2-0-stable"
+
+2. Install the gem using Bundler:
+
+        bundle install
+
+3. Copy & run migrations
+
+        bundle exec rails g spree_paypal_express:install
 
 ### Sandbox Setup
 
@@ -69,4 +77,4 @@ Copyright (c) 2013 Spree Commerce and contributors, released under the [New BSD
 [1]: http://www.fsf.org/licensing/essays/free-sw.html
 [2]: https://github.com/spree/better_spree_paypal_express/issues
 [3]: https://github.com/spree/better_spree_paypal_express/tree/master/LICENSE.md
-[4]: https://github.com/spree/spree_paypal_express
+[4]: https://github.com/spree/spree_paypal_express