Add GCP PubSub and alphabetize scalers list

README.md

@@ -72,15 +72,16 @@ KEDA performs two key roles within Kubernetes.  First, it acts as an agent to ac
 
 KEDA has a number of "scalers" that can both detect if a deployment should be activated or deactivated, and feed custom metrics for a specific event source.  Today there is scaler support for:
 
-* Kafka
-* RabbitMQ
-* Azure Storage Queues
-* Azure Service Bus Queues and Topics
 * AWS CloudWatch
 * AWS Simple Queue Service
+* Azure Event Hub
+* Azure Service Bus Queues and Topics
+* Azure Storage Queues
+* GCP PubSub
+* Kafka
 * Prometheus
+* RabbitMQ
 * Redis Lists
-* Azure Event Hub
 
 You can view other planned scalers [in our wiki and issue backlog](https://github.com/kedacore/keda/wiki/Scaler-prioritization).