Add a friendly welcome script, If there's no command being specified

[yukiisbored]

It would be nice to have a script that tells the user what they can do with this image, I was thinking about some example of commands that you can run and use. This can be added by making a entrypoint script to the docker image. The entrypoint script will be executed if there's no command specified when running the container.

[jayvdb]

A alternative proposal is #76, which adds an entrypoint which runs the typical coala --ci command. However, there is also the need for falling back to a helpful error message if running coala would not be useful, so this task is still valid.