Rework the guarantees exposed in the README

[o0Ignition0o]

As mentionned by @enkore in their awesome typo fix pull request, we might want to reconsider the "guarantees" section. What would make the most sense ?

Most of the items under this heading don't seem like guarantees to me, e.g. "completely asynchronous system design" is more of a feature that might imply certain guarantees. "Asynchronous program boundaries" also seems entirely like a feature to me, ditto for "dynamic supervision ..."

Originally posted by @enkore in https://github.com/bastion-rs/bastion/pull/173/files