Update README.md

Author: Ionut Trestian

README.md

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 # Amazon Cognito Identity SDK for JavaScript
 
+NOTE: If you are receiving a "_global.util.crypto.lib.randomBytes is not a function" error, it is an issue we are actively working on. For a workaround have a look at the issue: https://github.com/aws/amazon-cognito-identity-js/issues/646
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 You can now use Amazon Cognito to easily add user sign-up and sign-in to your mobile and web apps. Your User Pool in Amazon Cognito is a fully managed user directory that can scale to hundreds of millions of users, so you don't have to worry about building, securing, and scaling a solution to handle user management and authentication.
 
 We welcome developer feedback on this project. You can reach us by creating an issue on the