Add `arkade update` instructions like we have in k3sup

[Waterdrips]

Expected Behaviour

arkade update could print the upgrade instructions (like k3sup does)

Current Behaviour

no command

Possible Solution

check how k3sup does it, and use a similar approach
https://github.com/alexellis/k3sup/blob/master/cmd/update.go

[Waterdrips]

Derek add label: help wanted, skill/beginner, good first issue

[alexellis]

I'm surprised that I missed this, will add it in whilst I'm there.