add README.adoc for servicebus queue multibinders sample (#691)

spring-cloud-azure-samples/servicebus-queue-multibinders-sample/README.adoc

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+= Spring Cloud Azure Stream Binder for Multiple Service Bus Namespaces Code Sample
+
+This code sample demonstrates how to use the Spring Cloud Stream Binder for multiple Azure Service Bus namespaces.
+In this sample you will bind to two Service Bus namespaces separately through two queue binders.
+The sample app exposes REST APIs to receive string message. Then message is sent through Azure
+Service Bus to a `sink` which simply logs the message.
+
+Running this sample will be charged by Azure. You can check the usage and bill at https://azure.microsoft.com/en-us/account/[this link].
+
+== Connection string based usage
+
+1. Create two queues in different Service Bus namespaces. Please refer to https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-create-namespace-portal[Azure Service Bus].
+
+2. Update link:src/main/resources/application.properties[application.properties]
+
++
+
+[source%nowrap, properties]
+....
+
+# Configuration for default binder. Currently, the limitation is default binder must be provided
+# Fill the first Service Bus namespace connection string copied from portal
+spring.cloud.azure.servicebus.connection-string=[servicebus-namespace-1-connection-string]
+
+
+# Configuration for another binder
+spring.cloud.stream.binders.servicebus2.type=servicebus-queue
+# Set defaultCandidate to false
+spring.cloud.stream.binders.servicebus2.defaultCandidate=false
+spring.cloud.stream.binders.servicebus2.environment.spring.cloud.azure.servicebus.connection-string=[servicebus-namespace-2-connection-string]
+
+
+....
+
+[NOTE]
+====
+The *defaultCandidate* configuration item:
+
+Whether the binder configuration is a candidate for being considered a default binder, or can be used only when explicitly referenced. This allows adding binder configurations without interfering with the default processing.
+====
+
+
+== How to run
+1. Update stream binding related properties in link:src/main/resources/application.properties[application.properties]
+
++
+[source%nowrap, properties]
+....
+spring.cloud.stream.bindings.input.destination=[servicebus-queue-1-name]
+spring.cloud.stream.bindings.output.destination=[servicebus-queue-name-same-as-above]
+
+
+spring.cloud.stream.bindings.input1.destination=[servicebus-queue-2-name]
+spring.cloud.stream.bindings.output1.destination=[servicebus-queue-name-same-as-above]
+....
+
+2. Run the `mvn clean spring-boot:run` in the root of the code sample to get the app running.
+
+3.  Send a POST request to test the default binder
++
+....
+$ curl -X POST http://localhost:8080/messages?message=hello
+....
++
+
+4. Verify in your app's logs that a similar message was posted:
++
+....
+[1] New message received: 'hello'
+[1] Message 'hello' successfully checkpointed
+....
+
+5. Send another POST request to test the other binder
++
+....
+$ curl -X POST http://localhost:8080/messages1?message=hello
+....
++
+
+6. Verify in your app's logs that a similar message was posted:
++
+....
+[2] New message received: 'hello'
+[2] Message 'hello' successfully checkpointed
+....
+
+5. Delete the resources on http://ms.portal.azure.com/[Azure Portal] to avoid unexpected charges.