mention DB_CONNECTION env variable for testing

README.md

@@ -38,3 +38,14 @@ The Cloud Controller interacts with other core components of the Cloud Foundry p
 - Receives information from the Health Manager about applications
 - Subscribes to Service Gateways that advertise available services
 - Instructs Service Gateways to handle provisioning, unprovision, bind and unbind operations for services
+
+## Testing
+
+By default `rspec` will run test suite with sqlite3 in-memory database;
+however, you can specify connection string via `DB_CONNECTION` environment
+variable to test against postgres and mysql. Examples:
+
+    DB_CONNECTION="postgres://postgres@localhost:5432/ccng" rspec
+    DB_CONNECTION="mysql2://root:password@localhost:3306/ccng" rspec
+
+Travis currently runs 3 build jobs against sqlite, postgres, and mysql.