[web-php][PR #408] - Enhancement: Document platform requirement in composer.json

You can view the Pull Request on github: https://github.com/php/web-php/pull/408 Opened By: localheinz Pull Request Description: This pull request * [x] documents the platform requirement in composer.json 💁‍♂️ This project currently does not use composer to manage its dependencies, but it does not need to stay that way. Apart from using composer to manage dependencies, composer.json also serves documentation purposes. For a start, I propose to add composer.json with platform requirements - we can take it from there, if that make sense for you.