Add content for RxJs vs Promsies (#3044)

* Update 103-rxjs-vs-promises.md

* Update content/roadmaps/104-angular/content/101-rxjs-basics/103-rxjs-vs-promises.md

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

content/roadmaps/104-angular/content/101-rxjs-basics/103-rxjs-vs-promises.md

@@ -1 +1,11 @@
-# Rxjs vs promises
+# RxJS vs Promises
+
+In a nutshell, the main differences between the Promise and the Observable are as follows:
+
+- The Promise is eager, whereas the Observable is lazy,
+- The Promise is always asynchronous, while the Observable can be either asynchronous or synchronous,
+- The Promise can provide a single value, whereas the Observable is a stream of values (from 0 to multiple values),
+you can apply RxJS operators to the Observable to get a new tailored stream.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='blue' badgeText='Official Website' href='https://javascript.plainenglish.io/why-rxjs-rxjs-vs-promises-b28962771d68'>Why RxJS? RxJS vs Promises</BadgeLink>