Update readme.md

Author: akashsonowal

interview_prep/ml_system_design/readme.md

@@ -5,3 +5,4 @@
 - Personalized Search AutoSuggestion ([Flipkart](https://blog.flipkart.tech/building-personalized-autosuggestion-9e705d5bf5f8))
 - Video Recommendations ([YouTube](https://www.hellointerview.com/learn/ml-system-design/problem-breakdowns/video-recommendations))
 - Harmful Content ([Facebook](https://www.hellointerview.com/learn/ml-system-design/problem-breakdowns/harmful-content))
+- Notification Recommendation ([Reddit](https://sh.reddit.com/r/RedditEng/comments/1kqjhwf/an_indepth_look_at_the_notifications_recommender/))