fix 107

I might be missing the point but it seems to me that cognito is the best way to keep user uids across devices.
Using IAM users for each end user is overkill, not really needed.
With cognito, our application users, can use identity pools to get temporary access tokens.
Please correct me if I'm wrong.

Author: goncalodasilva

README.md

@@ -1516,8 +1516,8 @@ We are so thankful for every contribution, which makes sure we can deliver top-n
 
 - [ ] Create a user table in Amazon DynamoDB as key-value pairs of users and their devices. Use these keys as unique identifiers.
 - [ ] Use IAM-generated access key IDs for the users as the unique identifier, but do not store secret keys.
-- [ ] Implement developer-authenticated identities by using Amazon Cognito, and get credentials for these identities.
-- [x] Assign IAM users and roles to the users. Use the unique IAM resource ID as the unique identifier.
+- [x] Implement developer-authenticated identities by using Amazon Cognito, and get credentials for these identities.
+- [ ] Assign IAM users and roles to the users. Use the unique IAM resource ID as the unique identifier.
 
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