Add pronunciation key to README

[cab]

Motivation

Having a specific and standard pronunciation is helpful for discussing the project out loud.

Implementation

Following the suggestion in this document (paw-set-oh), add the IPA notation and the similar-word-based breakdown.

(I believe the author of that document is @paragonie-scott, so I'm fairly confident it's the right pronunciation :))

[paragonie-security]

Thank you! This will help make conversations easier :)