Create README.md

Author: ansd

README.md

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+# default-user-credential-updater
+
+This is a program that watches a RabbitMQ config file containing `default_user` and `default_pass` for changes.
+If the file changes, it updates the password in RabbitMQ.
+
+It is meant to be deployed as a sidecar container by https://github.com/rabbitmq/cluster-operator when HashiCorp Vault is enabled.
+
+The use case is as-follows:
+1. Default user password changes in Vault server.
+1. Vault agent sidecar places new password into `/etc/rabbitmq/conf.d/11-default_user.conf`.
+1. This sidecar (default-user-credential-updater) updates the password RabbitMQ server side by doing an HTTP PUT against the RabbitMQ Management API. This allows for default user password rotation without the need to restart RabbitMQ server.
+1. This sidecar copies new password to `/var/lib/rabbitmq/.rabbitmqadmin.conf` to be used by `rabbitmqadmin` CLI.
+
+See [vault-default-user](https://github.com/rabbitmq/cluster-operator/tree/main/docs/examples/vault-default-user) for an end-to-end example.