Document testing with RSpec (#495)

Author: eliotsykes

README.md

@@ -150,6 +150,36 @@ In the event you want to disable all tests without having to know the individual
 
 It is not required to send `SPLIT_DISABLE=false` to activate Split.
 
+To aid testing with RSpec, write `split_helper.rb` and call `use_ab_test(alternatives_by_experiment)` in your specs as instructed below:
+
+```ruby
+# Recommended path for this file is 'spec/support/split_helper.rb', and you will need to ensure it
+# is `require`-d by rails_helper.rb or spec_helper.rb
+module SplitHelper
+
+  # Usage:
+  #
+  # Force a specific experiment alternative to always be returned:
+  #   use_ab_test(signup_form: "single_page")
+  #
+  # Force alternatives for multiple experiments:
+  #   use_ab_test(signup_form: "single_page", pricing: "show_enterprise_prices")
+  #
+  def use_ab_test(alternatives_by_experiment)
+    allow_any_instance_of(Split::Helper).to receive(:ab_test) do |_receiver, experiment|
+      alternative =
+        alternatives_by_experiment.fetch(experiment) { |key| raise "Unknown experiment '#{key}'" }
+    end
+  end
+end
+
+RSpec.configure do |config|
+  # Make the `use_ab_test` method available to all specs:
+  config.include SplitHelper
+end
+```
+
+
 ### Starting experiments manually
 
 By default new A/B tests will be active right after deployment. In case you would like to start new test a while after