Update VZ_google_sheets readme for adding a new year

[gabrielwol]

bdit_data-sources/gis/school_safety_zones/README.md

Lines 66 to 68 in c3f1352

	## 4. Google Credentials
	Initially, a credential file (named `key.json` in the script) was required to connect to the Google Sheets to pull data. The google account used to read the Sheets is `bdittoronto@gmail.com`. First, Google Sheets API was enabled on the google account. Then, a service account was created so that we are not prompted to sign in every single time we run the script. Instructions on how to do that can be found at [Creating a service account](https://github.com/googleapis/google-api-python-client/blob/master/docs/oauth-server.md#creating-a-service-account). Go to the `Service accounts` page from there, select the `Quickstart` project and click on the `Search for APIs and Services` bar to generate credentials. Copy the credentials and paste it on a `key.json` file located in the same directory as the script. The `key.json` file should look something like this:

• what permissions need to be granted on sheet?
• correct email / service account?