docs: update readme with extensionIgnore option

Author: ahmadalfy

README.md

@@ -61,6 +61,13 @@ Default: `[]`
 Description: *list of classes for which the transformation will be ignored*  
 Example: `image.jpg => image.webp (instead of image.jpg.webp)`
 
+#### `extensionIgnore`
+
+Type: `Array<string>`  
+Default: `[]`  
+Description: *list of extension for which the transformation will be ignored*  
+Example: `extensionIgnore: ['svg']` will ignore transformation for images with the `svg` extension
+
 ### License [MIT](LICENSE)
 
 [npm]: https://img.shields.io/npm/v/posthtml-webp.svg