Update sqs.rst (#3423)

I think it would be nice to note that if you are using IAM roles on instances, that you can leave the access_key and secret_access_key blank, and let kombu get the keys from the meta data api. I spent a fair amount of time to figure out that someone else will do the hard work for me :)

docs/getting-started/brokers/sqs.rst

@@ -37,6 +37,10 @@ The login credentials can also be set using the environment variables
 :envvar:`AWS_ACCESS_KEY_ID` and :envvar:`AWS_SECRET_ACCESS_KEY`,
 in that case the broker URL may only be ``sqs://``.
 
+If you are using IAM roles on instances, you can set the BROKER_URL to:
+``sqs://`` and kombu will attempt to retrive access tokens from the instance
+metadata.
+
 .. note::
 
     If you specify AWS credentials in the broker URL, then please keep in mind