update README

README.md

-# Server side
+
+## Server side
+### Using binary executable
 Download the binary, make it executable and provide parameters \
-example : `./xcache-flusher -port 8080 -cachedir /tmp/testdir -token mysecretpassword` \
-help available with `./xcache-flusher -h`
+Example: `./xcache-flusher -port 8080 -cachedir /tmp/testdir01,/tmp/testdir02 -token mysecretpassword` \
+Help available with `./xcache-flusher -h`
 
 Server will listen on port 8080
 
-# Client side
+### Using Docker container
+Download the image: \
+`docker pull  gitlab-registry.in2p3.fr/bastien.gounon/xcache-flusher` \
+When running you must specify environment variables for configuration:
+- `FLUSHER_DIR`: comma-separated list of directories to flush
+- (opt) `FLUSHER_PORT`: port on which to listen, default 8080. Using default and redirecting a different host port via Docker is preferrable.
+- (opt) `FLUSHER_TOKEN`: string used to authenticate client requests \
+
+Example: 
+```
+docker run -p 54321:8080 --env FLUSHER_DIR=/xcache/data,/xcache/metadata --env FLUSHER_TOKEN=mysecretpassword -v /path/to/data:/xcache gitlab-registry.in2p3.fr/bastien.gounon/xcache-flusher
+```
+
+Docker-compose example:
+```
+version: '3'
+
+services:
+  xcache-flusher:
+    image: gitlab-registry.in2p3.fr/bastien.gounon/xcache-flusher
+    restart: always
+    volumes:
+      - /tmp/xcache:/xcache
+    environment:
+      - FLUSHER_DIR=/xcache/data,/xcache/metadata
+      - FLUSHER_TOKEN=mysecretpassword
+    ports:
+      - 54321:8080
+```
+
+## Client side
 Client just needs to call the DELETE HTTP operation on the server URL with method `flush`. Optionnally provide authentication token in the URL. \
 example:
   * with authentication: `curl -X DELETE http://127.0.0.1:8080/flush\?token\=mysecretpassword`