Add backup usage example for newbies

[LeavOA]

Took me a while to figure it out, so thought perhaps this could/should be added to the examples:

• the command comes first (e.g. backup)
• the arguments come second (e.g. -domain)

So for typical backup use case, the following command may be useful:

.\rfm.exe backup -domain [Duet ip or domain] ".\backup folder" "/"

This will backup everything (last slash means entire duet directory) on the duet at the [ip] into a subfolder called "backup folder" next to the executable (since the last argument is omitted, default is to backup everything)

Thought this could be useful to newbies like me.

Thanks for writing this very useful software!