README.md in gds-sso-13.4.0 vs README.md in gds-sso-13.5.0

- old
+ new

@@ -10,11 +10,11 @@
 - [publisher](https://github.com/alphagov/publisher)
 - [search-admin](https://github.com/alphagov/search-admin)
 
 ## Usage
 
-### Integration with a Rails 3+ app
+### Integration with a Rails 4+ app
 
 To use gds-sso you will need an oAuth client ID and secret for Signon or a compatible system.
 These can be provided by one of the team with admin access to Signon.
 
 Then include the gem in your Gemfile:
@@ -128,9 +128,21 @@
   # Pass in a caching adapter cache bearer token requests.
   config.cache = Rails.cache
 end
 ```
 
+If you are using a Rails 5 app in
+[api_only](http://guides.rubyonrails.org/api_app.html) mode this gem will
+automatically disable the oauth layers which use session persistence. You can
+configure this gem to be in api_only mode (or not) with:
+
+```ruby
+GDS::SSO.config do |config|
+  # ...
+  # Only support bearer token authentication and send responses in JSON
+  config.api_only = true
+end
+```
 
 ### Use in development mode
 
 In development, you generally want to be able to run an application without needing to run your own SSO server to be running as well. GDS-SSO facilitates this by using a 'mock' mode in development. Mock mode loads an arbitrary user from the local application's user tables:
 