proxmox-backupstorage: add some documentation

nodes/htz-hel/proxmox-backupstorage.toml

@@ -1,2 +1,19 @@
 hostname = "2a01:4f9:6b:2d99::2"
 dummy = true
+
+# How to install:
+# - Get server at Hetzner (no IPv4)
+# - Install latest proxmox compatible debian
+#   - RAID5
+#   - 50G for system
+#   - leave rest unpartitioned
+# - install zfs
+# - create additional partitions for remaining disk space
+# - create raidz on those partitions
+# - enable ipv6 forwarding
+# - install proxmox via apt
+
+# VM config:
+# - IPv6 only
+# - IP from the /64 hetzner gives us
+# - Gateway is the host itself, to work around the MAC filter hetzner uses