Documentation: Added a clean ES6 (ESM) usage example

[sdmnc]

Hi there!

I noticed in the README that you're looking for a good sample for ES6 (ESM) usage.
Since ESM is becoming the standard for Node.js, here is a clean and modern example
of how to use dotenv with the import syntax.

1. Update package.json

Add "type": "module" to your package.json to enable ES Modules:

{
  "type": "module",
  "dependencies": {
    "dotenv": "^16.4.5"
  }
}

2. Usage in index.js
The most elegant way to ensure variables are loaded before your app logic:

JavaScript
// This one-liner imports and executes config() immediately
import 'dotenv/config';

// Now you can access your variables
const dbUrl = process.env.DATABASE_URL;
const port = process.env.PORT || 3000;

console.log(`🚀 Server running on port ${port}`);
Why this approach?
Using import 'dotenv/config' is the cleanest way to guarantee that environment
variables are available even for other imported modules that might depend on them.

Hope this helps with the documentation!