Add example for predefined username

Author: ChunyiLyu

docs/examples/users/README.md

@@ -3,6 +3,9 @@
 This section contains 3 examples for creating RabbitMQ users.
 Messaging Topology Operator creates users with generated credentials by default. To create RabbitMQ users with provided credentials, you can reference a kubernetes secret object contains keys `username` and `password` in its Data field.
 See [userPreDefinedCreds.yaml](./userPreDefinedCreds.yaml) and [publish-consume-user.yaml](./publish-consume-user.yaml) as examples.
+From [Messaging Topology Operator v1.10.0](https://github.com/rabbitmq/messaging-topology-operator/releases/tag/v1.10.1), you can provide a username and reply on the Operator to generate its password for you.
+See [setUsernamewithGenPass.yaml](./setUsernamewithGenPass.yaml) as an example.
+
 Note that Messaging Topology Operator does not watch the provided secret and updating the secret object won't update actual user credentials.
 The User controller will generate a new secret from the provided secret named by appending the suffix `-user-credentials` to the username. For example, if your User is named `user-test` the controller will generate a secret `user-test-user-credentials`.
 If you wish to update User credentials, you can update the secret generated by the controller and then add a label or annotation to the User object to trigger a reconcile loop.

docs/examples/users/setUsernamewithGenPass.yaml

@@ -0,0 +1,19 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: user-creds
+type: Opaque
+stringData:
+  username: test
+---
+apiVersion: rabbitmq.com/v1beta1
+kind: User
+metadata:
+  name: set-username-gen-pass-example
+spec:
+  tags:
+  - management
+  rabbitmqClusterReference:
+    name: test # rabbitmqCluster must exist in the same namespace as this resource
+  importCredentialsSecret:
+    name: user-creds