Document how to subscribe an SQS queue

[rpaterson]

The AWS SNS "subscribe" call requires a parameter with an "Endpoint" property. The AWS docs say: For the sqs protocol, the endpoint is the ARN of an Amazon SQS queue. This does not work with fake_sns.

After much trial and error I finally figured out that fake_sns accepts the name of the SQS queue as the Endpoint instead of the SQS ARN. This is a deviation from the AWS SNS API.

In order to send a message to the SQS queue fake_sns needs to have the SQS queue's URL, and AFAICT there is no reliable way to figure this out from the ARN, so this isn't something that fake_sns can "fix". However please please at least document this behavior - it took me forever to figure it out.