doc: add readme license note (#21)

Add a note on license:

> The docker image is only valid for a short period

Author: cboudereau

README.md

@@ -9,7 +9,7 @@ Sybase docker image for local __dev only__.
 
 :warning: This image is not production ready and should be used for dev only
 
-For a dev usage a licence should be used and mounted in the container.
+For a dev usage a licence should be used and mounted in the container. The docker image is only valid for a short period.
 
 ## Run as docker container
 ```bash