Document the ability to pass rule structures at runtime to mk-session

[WilliamParker]

As discussed on the Slack channel with @mrrodriguez (see log) users are not required to define rules and queries with defrule and defquery. They can instead create rule data structures themselves at runtime and pass those data structures to mk-session, and they will then be compiled into a rules session. This should be more clearly documented.

Assigning to myself unless someone else has been working on this or has plans to.