Mention driver callbacks in the readme

Author: lutovich

README.md

@@ -68,6 +68,17 @@ driver.close();
 // Create a driver instance, for the user neo4j with password neo4j.
 var driver = neo4j.driver("bolt://localhost", neo4j.auth.basic("neo4j", "neo4j"));
 
+// Register a callback to know if driver creation was successful:
+driver.onCompleted = function() {
+  // proceed with using the driver, it was successfully instantiated
+};
+
+// Register a callback to know if driver creation failed.
+// This could happen due to wrong credentials or database unavailability:
+driver.onError = function(error) {
+  console.log('Driver instantiation failed', error);
+};
+
 // Create a session to run Cypher statements in.
 // Note: Always make sure to close sessions when you are done using them!
 var session = driver.session();