docs: add example for multiple compose files in README

Signed-off-by: Emilien Escalle <[email protected]>

Author: neilime

.github/ghadocs/branding.svg

@@ -215,3 +215,20 @@ steps:
         helloworld2
         helloworld3
 ```
+
+### Example with multiple compose files
+
+Specify multiple compose files to use with the `docker compose` command. This is
+useful when you have a base compose file and additional files for different
+environments or configurations.
+
+```yaml
+steps:
+  # need checkout before using compose-action
+  - uses: actions/checkout@v3
+  - uses: hoverkraft-tech/[email protected]
+    with:
+      compose-file: |
+        ./docker/docker-compose.yml
+        ./docker/docker-compose.ci.yml
+```