Add link to Flutter stack weekly

[kgdobariya]

Description

Added a link for weekly newsletter on new development and updates of Flutter universe.

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Checklist

• [x] I read How to contribute
• [x] I edited the SOURCE.md file only
• [x] Added a link to the repo in the PR

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Feel free to add this badge to your repository after it's accepted to awesome-flutter.

<a href="https://github.com/Solido/awesome-flutter">
   <img alt="Awesome Flutter" src="https://img.shields.io/badge/Awesome-Flutter-blue.svg?longCache=true&style=flat-square" />
</a>