Added an FAQ about array theory in CBMC

[akshayutture]

Issue #, if available:
Need an FAQ about CBMC's array-theory feature.

Description of changes:
Added an FAQ about CBMC's array-theory feature.

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[akshayutture]

Do you think it is necessary to provide users with a primer on array-theory?

[tautschnig]

At some point we need to consider where to draw the line: training material that both appeals to people completely new to verification (providing a gentle transition from testing to verification) as well as covering details about the decision procedure seems quite a stretch. Now I completely get that users are impacted by this, and quite likely it is users that are rather new to verification. But really I think this means we need to prioritize working on the array back-end.

[akshayutture]

Okay yeah that makes a lot of sense. The tutorial does seem to focus on new users and this point may be a little too detailed for new CBMC users. I'll close this PR then.

[jimgrundy]

Actually... I've been encountering the need to force the tool to use the array theory.
Rather than killing this I propose that we
1/ Add a section on Infrequently Asked Questions (aka advanced questions)
2/ Add a date to the material so people know when it was written

[jimgrundy]

Thanks