Add warning to `README` that suggests people to use other packages

[voxpelli]

It’s been suggested that the maintainers of this module are in agreement with the warning added to it on npmx.dev:

If that’s the case, may I suggest adding something like the following to the beginning of the README so that people who find this package through eg. GitHub or npm’s website will be equally informed:

> [!WARNING]
> The [community](https://e18e.dev/docs/replacements/) has flagged this package as having more performant alternatives. [Learn more](https://github.com/es-tooling/module-replacements/blob/main/docs/modules/dotenv.md)