Add content for Event Emitter (#1872)

* Update 101-event-emitter.md

* Update content/roadmaps/107-nodejs/content/104-nodejs-async-programming/101-event-emitter.md

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

content/roadmaps/107-nodejs/content/104-nodejs-async-programming/101-event-emitter.md

@@ -1 +1,7 @@
-# Event emitter
+# Event Emitter
+
+In Node.js, an event can be described simply as a string with a corresponding callback. An event can be "emitted" (or, in other words, the corresponding callback be called) multiple times or you can choose to only listen for the first time it is emitted.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='blue' badgeText='Official Website' href='https://nodejs.org/en/knowledge/getting-started/control-flow/what-are-event-emitters/'>What are Event Emitters?</BadgeLink>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.digitalocean.com/community/tutorials/using-event-emitters-in-node-js'>Using Event Emitters in Node.js</BadgeLink>