Add configuration documentation

README.md

@@ -20,7 +20,13 @@ Or install it yourself as:
 
 ### Configuration
 
-TODO: Write configuration instructions here
+You can add code to your initialization process to configure the integration with Buckaroo:
+
+    Buckaroo::Ideal::Config.configure(
+      partner_key: "Your Partner Key",
+      secret_key:  "Your Secret Key",
+      test_mode:   false # or true during development
+    )
 
 ## Usage