Add etcd as prerequisite for multi-user in gateway (#7022)

docs/multi-user/README.md

@@ -8,6 +8,7 @@ In this document we will explain in detail on how to configure multiple users.
 ### 1. Prerequisites
 - Install mc - [Minio Client Quickstart Guide](https://docs.minio.io/docs/minio-client-quickstart-guide.html)
 - Install Minio - [Minio Quickstart Guide](https://docs.minio.io/docs/minio-quickstart-guide)
+- Configure etcd (optional needed only in gateway or federation mode) - [Etcd V3 Quickstart Guide](https://github.com/minio/minio/blob/master/docs/sts/etcd.md)
 
 ### 2. Create a new user with canned policy
 Use [`mc admin policy`](https://docs.minio.io/docs/minio-admin-complete-guide.html#policies) to create canned policies. Server provides a default set of canned policies namely `writeonly`, `readonly` and `readwrite` *(these policies apply to all resources on the server)*. These can be overridden by custom policies using `mc admin policy` command.