Installing Longhorn on RD

[mook-as]

Somebody asked about installing Longhorn on Rancher Desktop; I'm not sure if this is useful for our docs (it's basically a retread of Longhorn docs — it's not like I looked at installing Longhorn before), but since I had it I might as well post it:

1. Installing Longhorn dependencies
   1. rdctl shell
   2. (From the shell) sudo su -
   3. apk add -U nfs-utils open-iscsi
   4. modprobe iscsi_tcp
   5. (Exit out of rdctl shell)
2. Install longhorn using the helm v3 method

Note: This does not work on Windows, because we can't load kernel modules there.