Add callbacks content (#1748)

* Update readme.md

* Update content/roadmaps/106-javascript/content/112-javascript-asynchronous-javascript/103-callbacks/readme.md

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

content/roadmaps/106-javascript/content/112-javascript-asynchronous-javascript/103-callbacks/readme.md

@@ -1 +1,6 @@
-# Callbacks
+# Callbacks
+
+A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.
+
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://javascript.info/callbacks'>Callbacks in JavaScript</BadgeLink>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://developer.mozilla.org/en-US/docs/Glossary/Callback_function'>Callback Functions</BadgeLink>