Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
In comments like this that are referring to an identifier in the code, I'd suggest putting it off with single quotes or something ('Names' is ...) in order to avoid having someone accidentally parse it as the English word.  You could also say something like "Here, 'Names' is" or "Below, 'Names' is" as a means of clarifying the switch from a "comments providing general statements" to a "comments on the code that's about to follow" mode.