update README

README.md

@@ -132,10 +132,23 @@ $ docker run -p 50051:50051 xain-fl
 
 ### Docker-compose
 
+The coordinator needs a storage service that provides an AWS S3
+API. For development, we use `minio`. We provide `docker-compose`
+files that start coordinator container along with a `minio` container,
+and pre-populate the appropriate storage buckets.
+
 #### Development
 
+To start both the coordinator and the `minio` service use:
+
+```shell
+docker-compose -f docker-compose-dev.yml up
+```
+
+It is also possible to only start the storage service:
+
 ```shell
-$ docker-compose -f docker-compose-dev.yml up
+docker-compose -f docker-compose-dev.yml up minio-dev initial-buckets
 ```
 
 #### Release