Add description for the -f flag

slides/containers/Container_Network_Model.md

@@ -307,6 +307,8 @@ Let's remove the `redis` container:
 $ docker rm -f redis
 ```
 
+* -f: Force the removal of a running container (uses SIGKILL)
+
 And create one that doesn't block the `redis` name:
 
 ```bash