Update readme to note MaxSpotInstanceCountExceeded

[vrivellino]

During testing, I came across this error from the request-spot lambda:

An error occurred (MaxSpotInstanceCountExceeded) when calling the RequestSpotInstances operation: Max spot instance count exceeded: ClientError
Traceback (most recent call last):
File "/var/task/handler.py", line 64, in handler
event['launch_subnet_id'], client_token)
File "/var/task/spoptimize/stepfns.py", line 106, in request_spot_instance
return spot_helper.request_spot_instance(launch_config, az, subnet_id, client_token)
File "/var/task/spoptimize/spot_helper.py", line 65, in request_spot_instance
Type='one-time', ClientToken=client_token)
File "/var/runtime/botocore/client.py", line 317, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/var/runtime/botocore/client.py", line 615, in _make_api_call
raise error_class(parsed_response, operation_name)
ClientError: An error occurred (MaxSpotInstanceCountExceeded) when calling the RequestSpotInstances operation: Max spot instance count exceeded

The solution was to request service limit increase via AWS Support.