Add idempotent example aws node express

[Nsupyq]

As described in issue #661, this PR creates a new example about what is idempotent function and how to create idempotent functions.

[mnapoli]

Hi, I think the idea mentioned in #658 (comment) is to create your examples on your own repository. This new example is quite specific, I'm not sure if duplicating the existing examples make sense.

Again, examples are to get users started easily. Besides idempotence, there are many many topics that could be covered to make an application production ready. If we had an example for each topic we would have a lot of examples.

[Nsupyq]

Ok, I will create examples on my own repository. How about adding a link of my repository in the README of aws-node-express-dynamodb-api?

[mnapoli]

You can add this to the community examples: https://github.com/serverless/examples#community-examples

[Nsupyq]

Thank you. I will propose a new PR.