Update - content for rxdart under flutter roadmap (#3352)

* Update 100-rxdart.md

* added links to 100-rxdart.md

* Update src/roadmaps/flutter/content/114-reactive-programming/100-rxdart.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

src/roadmaps/flutter/content/114-reactive-programming/100-rxdart.md

@@ -1 +1,5 @@
-# Rxdart
+# RxDart
+
+RxDart is a library for Dart that provides additional functionality for working with reactive programming, specifically with the Streams and Observables classes. It extends the standard Dart Streams API and provides additional features such as the ability to transform and combine streams, and to compose and chain streams together. In Flutter, RxDart is commonly used to handle asynchronous data streams and user interactions in a more efficient and elegant way.
+
+- [RxDart Official Docs](https://pub.dev/documentation/rxdart/latest)