Provide Guidance on how customers can upgrade Azure PowerShell Commandlets

There was a [thread](https://twitter.com/i/web/status/811940509295976449) on twitter in which a customer was running into an issue where the version of the Azure PowerShell commandlets on their local machine was a different version than what was in Azure Functions. We should provide some guidance on how customers can update the version of the Azure PowerShell Tools which is available in Azure Functions.

We will want to make a separate topic under the [Azure Functions Developer Guide](https://docs.microsoft.com/en-us/azure/azure-functions/functions-reference) (new language node for PowerShell)