Add content for callback (#1830)

* Update 104-callbacks.md

* Update content/roadmaps/107-nodejs/content/104-nodejs-async-programming/104-callbacks.md

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

content/roadmaps/107-nodejs/content/104-nodejs-async-programming/104-callbacks.md

@@ -1 +1,7 @@
-# Callbacks
+# Callbacks
+
+Node.js, being an asynchronous platform, doesn't wait around for things like file I/O to finish - Node.js uses callbacks. A callback is a function called at the completion of a given task; this prevents any blocking, and allows other code to be run in the meantime.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='blue' badgeText='Official Website' href='https://nodejs.org/en/knowledge/getting-started/control-flow/what-are-callbacks/'>What are callbacks?</BadgeLink>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://nodejs.dev/en/learn/javascript-asynchronous-programming-and-callbacks/'>Asynchronicity in Programming Languages</BadgeLink>