Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
Rather than "limitation" how about "restriction"?

Though the following is much wordier, I might re-phrase this as:
"This restriction exists because Chapel binds arrays to their domains for their lifetime to support semantic reasoning by the compiler and users.  Extending an array via indexing modifies its domain's index set, which would in turn modify [affect?] the keys+values of any other arrays sharing that domain as well.  We consider this behavior to be too surprising to support ("I modified Ages, why did Scores change as well?").  Thus, to extend an array which shares its domain with other arrays, the user is required to modify the domain directly -- the rationale being that it should be less surprising that modifying a domain would affect all arrays defined over that domain.

Feel free to try to tighten that up or clarify it further, but I think we should err on the side of being verbose/complete/clear here since I think this may become a FAQ over time.