Add docs for authing to Azure with service principal

[joeyaiello]

We have a basic demo that authenticates into Azure using a service principal. We should doc this example and explain how/why it works.

[TylerLeonhardt]

Eamon's doing this, yeah?

[TylerLeonhardt]

@eamonoreilly 👋

[joeyaiello]

Not sure that this is actually needed once we have MSI support, throwing it on the backlog.

[sspage]

This is in direct conflict with #112 where you say you are removing support for service principals. Which is it? Hope you are not removing it or will provide guidance on another method for AzureRM auth.

[TylerLeonhardt]

Please see my response in #112.

[asavaritayal]

/cc @ggailey777 as FYI

[anirudhgarg]

We have MSI support, closing this out.