Replace instructions for hello-world practice exercise

[mikedamay]

Hello-world has the instructions for the Bob exercise (and presumably other files). Check with @ErikSchierboom that an introductory hello-world exercise is required and if so amend the files to provide a simple return of a "hello world" string.

[ErikSchierboom]

We will be doing a rework of all the hello-world exercises, so I'd not work on this for a while until we have a spec defined.