Update Preface to note CC-SA license and terms

[aslakknutsen]

I'm guessing some standard O'Reilly copyright notice thingga-ma-jigg to protect front cover / logos etc will be included elsewhere?

[ALRubinger]

We're in production now, so any edits have to be made through them and not into the Atlas repo.

Production will review and revise our stuff to their needs too.