[Kettle] Add some docs on Kettle and PubSub

[MushuEE]

Describe auth needs to PubSub changes that I made this morning.

For reference the failure was as follows:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/google/cloud/pubsub_v1/_gapic.py", line 40, in <lambda>
    fx = lambda self, *a, **kw: wrapped_fx(self.api, *a, **kw)  # noqa
  File "/usr/local/lib/python3.6/dist-packages/google/pubsub_v1/services/subscriber/client.py", line 1012, in pull
    response = rpc(request, retry=retry, timeout=timeout, metadata=metadata,)
  File "/usr/local/lib/python3.6/dist-packages/google/api_core/gapic_v1/method.py", line 145, in __call__
    return wrapped_func(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/google/api_core/retry.py", line 286, in retry_wrapped_func
    on_error=on_error,
  File "/usr/local/lib/python3.6/dist-packages/google/api_core/retry.py", line 184, in retry_target
    return target()
  File "/usr/local/lib/python3.6/dist-packages/google/api_core/grpc_helpers.py", line 59, in error_remapped_callable
    six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "<string>", line 3, in raise_from
google.api_core.exceptions.PermissionDenied: 403 User not authorized to perform this action.

/area kettle
/cc @spiffxp @amwat

[spiffxp]

==================== Test output for //hack:verify-spelling:
Validating spelling...
./kettle/README.md:114:19: "permisions" is a misspelling of "permission"
ERROR: bad spelling, fix with hack/update-spelling.sh

[spiffxp]

A few questions

[spiffxp]

/test pull-test-infra-integration
#20498 (comment)

[MushuEE]

@amwat able to LGTM?

[spiffxp]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MushuEE, spiffxp

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• kettle/OWNERS [MushuEE,spiffxp]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment