Document --tag flag and how it works

[alexellis]

Find a place in the CLI docs to document how the --tag flag works.

Remember this covers build/push and deploy - do we really want to duplicate the text in each part - or could it be a sub-page of the CLI heading?

openfaas/faas-cli#465

[alexellis]

@MrTinD do you want to take a shot at this? We can review the wording and guide some guidance after the technical parts are in? You'll find instructions in the README for how to test locally.

[martindekov]

Yeah I'm on it.