Docs: what problem is NullBroadcasts solving

(I know documentation is under construction)

The current documentation describes the package, but does really mention where/why one would use `NullBroadcasted.

It'd be useful to add a line to provide more motivation for the package.

> This package defines NullBroadcasted(), which can be added to, subtracted from, or multiplied by any value in a broadcast expression without incurring a runtime performance penalty. *This can be useful for X*