doc: add introduction sentence for CJS

PR-URL: nodejs/node#42491
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>

doc/api/modules.md

@@ -6,7 +6,11 @@
 
 <!--name=module-->
 
-In the Node.js module system, each file is treated as a separate module. For
+CommonJS modules are the original way to package JavaScript code for Node.js.
+Node.js also supports the [ECMAScript modules][] standard used by browsers
+and other JavaScript runtimes.
+
+In Node.js, each file is treated as a separate module. For
 example, consider a file named `foo.js`:
 
 ```js