More detailed instructions on fetching Google creds

[ryanhalliday]

Getting the Google Credentials for someone that hasn't done it before can be a bit of a pain.

Such as you have to add this API and not this API, then invite the email address found here to your GA account after setting up the Service account.

Could the documentation cover this in some more depth?

[mikecook]

If you turn that into a PR (and verify it's still correct) I'll merge it.