Readme Step 4 duplicates SP creation from Step 3, bypassing certificate

[yevster]

This issue is for a: (mark with an x)

- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

Steps 3 and 4 each produce a different service principal

3. Create an Azure service principal, using
   Azure CLI,
   PowerShell
   or Azure Portal.
   Note that if you wish to authenticate with the certificate authenticator the certificate should be saved locally.

4. Use an authentication file to authenticate to the Azure management plane.

The document in step 4 contains no information for showing an auth file for an existing certificate, so it is not helpful following step 3.

The simplest fix would be to replace both steps with a single CLI command:

 az ad sp create-for-rbac --name keyvaultsample  --create-cert --sdk-auth > auth.file

A PR is forthcoming.

OS and Version?

Ubuntu 18.04 LTS