docs: Downloading my service account key is confusing

[ryanseys]

Instructions in README say:

Click on "Create new client ID" to create a new service account. Once the account is created, click on "Generate new JSON key" to download your private key.

When I click on 'Create new Client ID', a JSON file is immediately downloaded to my machine. By clicking 'Generate new JSON key' I just create a second one and download it too... this is confusing because now I have two JSON files.

[stephenplusplus]

Going to "move" this to #144 for the readme rewrite.