[zdict] Add a FAQ to the top of zdict.h

[zdict] Add a FAQ to the top of zdict.h

The FAQ covers the questions asked in Issue #2566. It first covers why
you would want to use a dictionary, then what a dictionary is, and
finally it tells you how to train a dictionary, and clarifies some of
the parameters.

There is definitely more that could be said about some of the advanced
trainers, but this should be a good start.