Update to npm usage documentation.

[Mod5ied]

Using the { "scripts": { "dev": "nodenv .", "test": "nodenv -E test/.env test/test.js" } } seems rather unclear and not very comprehensive.
I tried registering the command for multiple windows users and errors arose. Subsequently, I found a more straightforward approach to running the package from the scripts:
"scripts": { "dev": "**node-env-run app.js**", }
Using this format proved very simple to understand and easy to test out.
Please do well to consider making the change, or assign me the privilege to do so. Thanks.