Add notes on how to update the Google sheets credentials

Add notes on how to update the Google sheets credentials

I had to reverse engineer this a little bit prior to the 2023 Mystery
Hunt, (by trying to use the "New Hunt" button in Turbot and finding an
error of "invalid_grant: Bad Request" coming from
gsheets_creds.refresh(Request()) in the CloudWatch logs for the turbot
lambda).

The new instructions here tell the admin how to update the Gsheets
credentials (by running ./gsheets-authenticate.py) and how to take the
value that results (in the .gsheets-pickle.base64 file) and put it
into the SSM parameter named GSHEETS_PICKLE_BASE64 in ASM Systems
Manager in the AWS interface.