Adding note about iframing Authorization

[yass-aircall]

PR Title

Description

If it's a bug, list the steps to reproduce If it's a feature, list things to check and what's impacted

Submission

• [ ] Your branch is based on master
• [ ] Your code is unit tested
• [ ] CircleCI builds are green (coverage and lint)
• [ ] Add team "CTI" as "Reviewer" for the PR
• [ ] Name your PR with the convention fix|feat|impr(<subject>): description.