all: document how to pass in Spanner configuration variables

[odeke-em]

Currently while creating the Django adapter, there are two ways to parse and create a connection:

a) With a dict with SPANNER_URL defined in the database configuration as per

{
    'SPANNER_URL': 'cloudspanner:/projects/appdev-soda-spanner-staging/instances/django-dev1/databases/db1?instance_config=projects/appdev-soda-spanner-staging/instanceConfigs/regional-us-west2'
}

b) An undocumented way with variables in the database configuration:

{
     'AUTO_COMMIT': True,
     'NAME': 'db1',
     'INSTANCE': 'instance1',
     'INSTANCE_CONFIG': '...', # Currently unimplemented for the dict reading
}

In b) INSTANCE_CONFIG parsing is not yet implemented, PR coming up for that shortly to unblock @timgraham.