docs/Clients.md: Add documentation about clients supporting pleroma
- Twidere: development slowed down, might be active again, will not be in the list for now
- Tootle (iOS): needs to go further/check again
- Pawoo: I’ll try with my pixiv account to see if there is some kind of contact. // Seems like there is only support
- Tootle (GTK3): has few modifications for supporting pleroma, need confirmation, not so sure if maintained