Document Plasma Mobile installation

From Matrix chat:

1. Flash pureos
2. apt install gnupg2
3. apt-key adv --keyserver keyserver.ubuntu.com --recv-keys CF319275556E40EC
4. Add deb https://repo.kaidan.im/debpm/ buster main devkit debline in /etc/apt/sources.list
5. apt update
6. Disable phosh.service
7. Install kwin-wayland kwin-wayland-backend-drm plasma-phone-components plasma-settings plasma-phone-settings simplelogin (I also removed gnome packages to make space as I haven't figured out how to increase size of rootfs)
8. Add export ETNA_MESA_DEBUG=no_supertile to /etc/profile.d/etnaviv.sh
9. systemctl enable simplelogin and reboot
10. modify /lib/systemd/system/simplelogin.service to use purism user instead of phablet

Make sure kwin packages are from devkit component ^^ if they're from the debian/pureos it won't work.