Add note about ESM

Author: sindresorhus

readme.md

@@ -44,6 +44,10 @@ We accept contributions for commonly used modern file formats, not historical or
 npm install file-type
 ```
 
+**This package is a ESM package. Your project needs to be ESM too. [Read more](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c).**
+
+If you use it with Webpack, you need the latest Webpack version and ensure you configure it correctly for ESM.
+
 ## Usage
 
 #### Node.js