Update the README to add guide how to create an admin user.

README.md

@@ -11,6 +11,16 @@
   update the PHX_HOST to your domain name
 - run `docker compose up -d`
 - run migrations `docker exec -it dbi bin/migrate`
+- open the url (`PHX_HOST`) in a browser
+  register a user
+- run the commands to promote an user to the admin.
+  change `admin@example.com` to your email.
+  ```shell
+  docker exec -it dbi bin/decentralised_book_index remote
+  DecentralisedBookIndex.Accounts.User.get_by_email!("admin@example.com", authorize?: false) \
+  |>  DecentralisedBookIndex.Accounts.User.set_role(:admin, authorize?: false)
+  ```
+  close the terminal with `Ctrl + c` and `a`.
 
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