document update to create-worker --countries argument

dev/README.md

@@ -5,8 +5,8 @@ This docker-compose configuration to be used **only** for development purpose.
 ### backend
 
 This container is a backend web server, linked to its database.
-It provides a command `mirrors-qa-backend` to simplify tasks like updating of mirrors.
-Run `mirrors-qa-backend --help` in the container to see the sub-commands and options.
+It provides sub-commands to simplify tasks like updating of mirrors.
+Run `mirrors-qa-backend --help` in the container to see the various sub-commands and options.
 
 ### postgresqldb
 
@@ -64,8 +64,9 @@ docker compose --profile worker up --build
     The key name `id_rsa` is used as a bind mount in the compose file.
 
 - Assuming the backend service is up (`docker compose up backend`), create a worker and assign them a list of countries to test for.
-    If an existing worker is responsible for those countries, they are assigned to the new worker. In this example, we create a worker
-    named `test`.
+    If no countries are provided, all available countries wiil be assigned to the worker.
+    In this example, we create a worker named `test` to test for mirrors in France, United States and Canada using the private key file
+    named `id_rsa`.
     ```sh
     docker exec -i mirrors-qa-backend mirrors-qa-backend create-worker --countries=us,fr,ca test < ./id_rsa
     ```