docs: explain how this flake is intended to be used

Author: 99linesofcode

README.md

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+# Devshell-PHP
+
+A nix flake based development environment for Laravel with `PNPM` and `Xdebug` support. The goal is to package everything relevant for running your development environment. Installing packages, running a front-end watcher, linters, formatters, composer, etc. Everything else should be abstracted away into dedicated Docker containers so as to match production as closely as possible.
+
+## How to use
+
+This devshell is meant to be pulled into your project as a git submodule. The idea is to manage changes in a separate and centralized repository. If different versions need to be maintained then those should be stored on separate branches.
+
+To use `devshell-php` and assuming you are using `nix-direnv`:
+
+1. Add it as a git submodule `git submodule add https://github.com/99linesofcode/devshell-php ./devshell`
+1. Define a `.envrc` in your project root folder `echo use flake ./devshell >> .envrc`
+3. Run `direnv allow`
+1. Add `.direnv/` to your `.gitignore`
+
+## Contributing
+
+Please review our [Contribution Guidelines](https://github.com/99linesofcode/.github/blob/main/.github/CONTRIBUTING.md).
+
+## Code of Conduct
+
+In order to ensure that the community is welcoming to all, please review and abide by the [Code of Conduct](https://github.com/99linesofcode/.github?tab=coc-ov-file).
+
+## Security Vulnerabilities
+
+Please review [our security policy](https://github.com/99linesofcode/.github?tab=security-ov-file) on how to report security vulnerabilities.
+
+## License
+
+This software is open source and licensed under the [MIT license](https://github.com/99linesofcode/.github?tab=MIT-1-ov-file)