doc: add qrencode to brew install instructions

Summary:

Build was missing qr code(qrencode libs) support

Backport of core PR12850
https://github.com/bitcoin/bitcoin/pull/12850/files

Depends on D3909.

Test Plan: Read it, test it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3911