Add to FAQ: how do I join testflight

[alltheseas]

https://damus.io/purple/

[alltheseas]

This remains open and unclear.

Proposed solution: add reference/clear instructions how to join testflight for purple members

https://damus.io/note1z32udd00qrh8fwxzyqayyy2975jszxxs8436xs4nj5qpfu4ye0yqf3jz8p