Add driver.close() to the usage example

Author: lutovich

README.md

@@ -66,6 +66,7 @@ driver.close();
 ```javascript
 
 // Create a driver instance, for the user neo4j with password neo4j.
+// It should be enough to have a single driver per database per application.
 var driver = neo4j.driver("bolt://localhost", neo4j.auth.basic("neo4j", "neo4j"));
 
 // Register a callback to know if driver creation was successful:
@@ -148,6 +149,9 @@ if (success) {
   console.log('rolled back');
   tx.rollback();
 }
+
+// Close the driver when application exits
+driver.close();
 ```
 
 Subscriber API allows following combinations of `onNext`, `onCompleted` and `onError` callback invocations: