Document the requirements on targets

[davidwagner]

I have heard, but cannot confirm, that no alternative should be a substring of any other, otherwise it will trigger lots of false positives for the cheat detection. For example, # DEFINE VNAME Customer CustomerName is apparently no good. Is this correct?

If this is correct, this should be documented in exam-write/README.md. The restrictions on choosing targets and alternatives needed to ensure the cheat detection works well should be documented there.