Add to quick start documentation: once you create a case, do not change the sandbox

A seemingly unintuitive aspect of CESM/cime is that, once you have created a case from a particular directory, it is very important not to change the code checked out in that directory until you are completely done with your case. Is that documented somewhere? I don't see it in the quick start guide, and that seems like a good place to include a warning about this. Maybe we can have a section at the end like "Important points to bear in mind"?