documentation for policy.json

Hello,
Thanks for the job you've done! Chalice is the very easy framework for newcomers and flask developers.

I've been learning the framework only for 2 weeks and regularly getting errors related with aws policies. The official content about the policies is minimal. And auto-policy can't do this job by the right way if you want to add more than just a 'hello world' function.

Run **chalice gen-policy** (chalice==1.13.1) returns only:
```
{
"Version": "2012-10-17",
"Statement": []
}
```
It absolutely doesn’t help me in setting up the policy of an application that is already using dynamodb, sns and s3.

Is there an extended documentation describing the policy.json or examples for regular cases, list of actions and values.

Start with the policy.json output from chalice gen-policy and review the reported DynamoDB, SNS, and S3 use cases. Compare those cases with the AWS policy guidance and define the documentation or examples needed; done means newcomers can find valid policy.json patterns, actions, and values for regular applications.

Written by the indexing model from the issue text.