Update README.md

Author: suryarajendhran

README.md

@@ -121,7 +121,9 @@ The library has no dependencies, works with plain old HTML/CSS/JS.
 <!-- USAGE EXAMPLES -->
 ## Usage
 
-### Get a CSS Rule
+### Usage as an ES6 module
+
+#### Get a CSS Rule
 
 Use getCSSRule() to search your document's stylesheets for a [CSS Rule](https://developer.mozilla.org/en-US/docs/Web/API/CSSRule).
 
@@ -133,7 +135,7 @@ const CSSRule = getCSSRule('.text-base');
 // else returns false.
 ```
 
-### Modify a CSS Rule
+#### Modify a CSS Rule
 
 The modifyCSSRule() function makes it easier to modify a [CSS Rule](https://developer.mozilla.org/en-US/docs/Web/API/CSSRule) with a single function call.