New instructions for Nextcloud Snap (#345)

* README: Revers Proxy configuration with Nextcloud Snap

Signed-off-by: Pilzinsel64 <pilzinsel64@gmx.de>

* README: update snap notes

Signed-off-by: Pilzinsel64 <pilzinsel64@gmx.de>

* README: polish up

Signed-off-by: Pilzinsel64 <pilzinsel64@gmx.de>

---------

Signed-off-by: Pilzinsel64 <pilzinsel64@gmx.de>