> Please allow access on the next pop-up to discover address book users reachable from Riot.

Please don't say please to make people perform questionable privacy violations. How about:

"If you want Riot to determine which of your contacts also use Matrix and to easily enable you to talk to them via Riot, you can allow Riot to access your contact list.

Note: This will upload all your contacts' details, as stored on your phone, including addresses, birthdays, notes, and more if available to matrix.org. Here is the privacy policy."

Or something like that, whatever it really does.

Disclaimer: I like matrix :)))