Add 5 developer newsletters with sponsorship availability

Quastor (backend engineering), iOS Dev Weekly, Last Week in AWS,
CSS Weekly, and Kotlin Weekly. Fills gaps in iOS, cloud, CSS, Kotlin,
and backend coverage.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: nimrodkra

README.md

@@ -67,6 +67,11 @@ The usual marketing tactics don't work on developers. They decide what to use by
 - [Hacker Newsletter](https://hackernewsletter.com) - Weekly curated digest of the top stories from Hacker News.
 - [Software Lead Weekly](https://softwareleadweekly.com) - Weekly newsletter for engineering managers and technical leaders.
 - [This Week in React](https://thisweekinreact.com) - Weekly newsletter covering React, React Native, and the surrounding ecosystem.
+- [Quastor](https://www.quastor.org) - Backend engineering newsletter covering system design, data engineering, and distributed systems.
+- [iOS Dev Weekly](https://iosdevweekly.com) - Hand-picked roundup of the best iOS development links delivered weekly.
+- [Last Week in AWS](https://www.lastweekinaws.com/newsletter) - Weekly newsletter covering AWS news, changes, and cloud infrastructure updates.
+- [CSS Weekly](https://css-weekly.com) - Weekly roundup of CSS articles, tutorials, experiments, and tools.
+- [Kotlin Weekly](http://www.kotlinweekly.net) - Weekly newsletter covering Kotlin development, libraries, and ecosystem news.
 
 ## Content Marketing Platforms