Add design notes to @odk/xpath README

Per [this review comment](https://github.com/eyelidlessness/odk-web-forms/pull/3#discussion_r1365994714):

> Might be good to augment with some of the design considerations from the PR description and some things we discussed about deferring to native like the current Enketo evaluator does vs not

I decided to merge and come back to this later, but I agree it would be good to have. I think in general we'll want the same in other READMEs and perhaps even something at the root.