Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
Ditto the above: Are other op= operators supported on the arrays, and do they have similar limitations.

If only these two operators are supported on arrays then I'd remove "arrays" from the initial comment about operators on "domains and arrays" and make that first section be about domain set operators and then start this new section on the topic of array set operators, specifically calling out that it's only a subset of the domain operators.

(Sorry that I can't seem to keep this straight).

I might wrap up this file with a "future directions" statement -- the main thing I can think of to mention is that associative domains cannot today be distributed across multiple locales, but that we have a prototype domain map that would support this and can put more effort into adding the feature if users request it.  (I know some who would).  This is the main/obvious question that I think is not answered by the primer which I think someone would have.