Write public documentation how to use New-TestResources.ps1

[heaths]

We need to document how to use New-TestResources.ps1 (though we should move it first; see Azure/azure-sdk-tools#358), including how to create a service principal until we do it ourselves (see Azure/azure-sdk-for-net#9060).

[kurtzeborn]

@danieljurek, with the last PR (Azure/azure-sdk-tools#410), all that's left here is updating the contributing.md files to reference the script and it's documentation.