The aws-cli documentation and command line help text have not been updated yet to include the syntax for subscribing an AWS Lambda function to an SNS topic, but it does work!
Here’s the format:
aws sns subscribe \
--topic-arn arn:aws:sns:REGION:ACCOUNT:SNSTOPIC \
--protocol lambda \
--notification-endpoint arn:aws:lambda:REGION:ACCOUNT:function:LAMBDAFUNCTION
where REGION, ACCOUNT, SNSTOPIC, and LAMBDAFUNCTION are substituted with appropriate values for your account.
For example: