feat(subjects): add replay subject description (#308)

subjects/replaysubject.md

@@ -2,6 +2,24 @@
 
 ## "Replays" or emits old values to new subscribers
 
+### Why use `ReplaySubject`?
+
+This subject variation shines when you want subscribers to have access to previous values emitted, even if they subscribe after those values have been sent. Think of it as a Tivo or DVR for observables. Missed the start of the show? No worries! With a `ReplaySubject`, you can still catch up on what you missed.
+
+For a real-world example, consider a chat application. When a user joins a chatroom, they might want to see the last few messages, not just new ones. A `ReplaySubject` can hold onto a specified number of the most recent messages and display them to the user when they join.
+
+Now, how does it compare with `BehaviorSubject` or a plain `Subject`?
+
+- **[`BehaviorSubject`](behaviorsubject.md)** always requires an initial value and only stores the most recent value. Subscribers will get that latest value upon subscription. It's like walking into a movie and only catching the most recent scene.
+
+- **[`Subject`](subject.md)**, on the other hand, doesn't give new subscribers any previously emitted values. It’s the traditional live broadcast. If you're late, you miss out.
+
+- **`ReplaySubject`** can remember more than just the last scene. Depending on how it's configured, it might replay the entire movie or just the last few scenes.
+
+However, be cautious. `ReplaySubject` can potentially use more memory since it's storing multiple values. Ensure that you specify a limited buffer size if you're concerned about memory consumption.
+
+In conclusion, if you want the ability to replay a series of previous emissions to new subscribers, `ReplaySubject` is your go-to. If you only care about the most recent emission (with an initial value), opt for `BehaviorSubject`. And if you want a basic, no-frills broadcasting mechanism where late subscribers miss prior emissions, stick with a plain `Subject`.
+
 [![Ultimate RxJS](https://drive.google.com/uc?export=view&id=1qq2-q-eVe-F_-d0eSvTyqaGRjpfLDdJz 'Ultimate RxJS')](https://ultimatecourses.com/courses/rxjs?ref=4)
 
 ### Examples