Add Delta Chat

[SedjamTan]

Delta Chat is possibly originally based on Signal.

1. Delta Chat is under GPLv3.
2. Source Code available
3. Privacy Policy indicates that apart from push notifications (consent-based), usage statistics (opt-in, consent-based), and call fallback relay and mini-app collection server (both are run by default), the developers do not collect or store any personal data or messages from you. Do note that they indicate that there are third-party processors that process your data, however I think it's minimal and probably from the optional features of Delta Chat.
4. Possibly no proprietary elements.
5. It is mostly stable.
6. Still in active development. Note that first commit at August 14, 2018 is code from the Signal app. <--- need help with this
7. Has a wiki listed in their github and in their website.
8. Free of charge

[offa]

Thanks for your PR. I wonder why this wasn't added earlier!? 🤔