add retrospective resources

suggested_study/retrospectives.md

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+# Retrospectives
+
+- [Retrospective Academy](https://www.retrium.com/ultimate-guide-to-agile-retrospectives/intro)
+- [Do's and don't's](https://www.inloox.com/company/blog/articles/do-s-and-don-ts-how-to-conduct-effective-retrospectives/)
+- [3 tips](https://echometerapp.com/en/retrospective-action-items-tips-examples/)
+- [Remote Retrospectives](https://www.atlassian.com/blog/teamwork/run-retrospective-distributed-team-fun)