add info about the toolbox image

README.md

@@ -11,6 +11,8 @@ A minimal deployment of allgo consists of 4 docker images:
 - **allgo/mysql**: the mysql database server
 - **allgo/controller**: the manager for user docker containers
 - **allgo/ssh**: the ssh frontend (giving access to the sandboxes)
+- **allgo/toolbox**: an image containing a set of commands (scp, vi, nano,
+  less, ...) to be mounted in the user sandboxes
 
 These images may be deployed multiple times to implement multiple independent
 environments (eg: production, qualification, ...).