Add a link to the #231 issue as searching the issue by hand is cumbersome

There is no need to send a HEAD request to the server since we now
bundle external_api.js.

Co-authored-by: Christophe HAMERLING <chamerling@linagora.com>
Co-authored-by: Klemens Arro <klemens.arro@admcloudtech.com>
Co-authored-by: Goran Urukalo <goran.urukalo@teletrader.com>

Update frame-ancestors note.

It was not clear that the cask formula was available for this promising app: https://github.com/Homebrew/homebrew-cask/blob/master/Casks/jitsi-meet.rb

Fixes: https://github.com/jitsi/jitsi-meet-electron/issues/124