docs: add CONFIG_REFERENCE.md and CONTRIBUTING.md; update INTEGRATION and README

CONTRIBUTING.md

@@ -0,0 +1,75 @@
+# Contributing
+
+Thanks for contributing! This document explains how to run tests, linting, and the suggested workflow for changes.
+
+## Quick dev setup
+
+1. Install dependencies (developer machine):
+
+```bash
+git clone <repo-url> /path/to/upload-logger
+cd /path/to/upload-logger
+composer install --no-interaction --prefer-dist
+```
+
+2. Run unit tests and static analysis:
+
+```bash
+vendor/bin/phpunit --configuration phpunit.xml
+vendor/bin/phpstan analyse -c phpstan.neon
+```
+
+3. Run PHP lint across the project (example):
+
+```bash
+find . -name '*.php' -not -path './vendor/*' -print0 | xargs -0 -n1 php -l
+```
+
+## Branching & PR workflow
+
+- Create a feature branch from `main` (or `master` if your repo uses it):
+
+```bash
+git checkout -b feature/short-description
+```
+
+- Make small, focused commits with clear messages. Example:
+
+```
+Add CONFIG_REFERENCE.md mapping configuration options
+```
+
+- Push and open a pull request to `main`. Provide a short description of the change and mention testing steps.
+
+## Tests and CI
+
+- The repository uses GitHub Actions to run PHPUnit and PHPStan on supported PHP versions. Ensure tests pass locally before opening a PR.
+- If you add new functionality, provide unit tests in `tests/` and update `phpunit.xml` if needed.
+
+## Smoke tests
+
+- A basic smoke harness exists under `tests/smoke/`. To run locally:
+
+```bash
+php -S 127.0.0.1:8000 -t tests/smoke/public -d auto_prepend_file=$(pwd)/upload-logger.php
+# then POST files with curl or a test client
+```
+
+## Coding style
+
+- Keep changes minimal and consistent with existing code. Avoid reformatting unrelated files.
+- Follow PSR-12 style where practical for new PHP code.
+
+## Adding docs
+
+- For user-facing changes, update `README.md`, `docs/INSTALLATION.md` and `INTEGRATION.md` accordingly. Prefer short, copy-paste examples for operators.
+
+## Security disclosures
+
+- If you find a security vulnerability, do not open a public issue. Contact maintainers privately and include reproduction steps.
+
+## Contact
+
+- Open an issue or a PR on GitHub; maintainers will review and respond.
+
+Thank you for helping improve Upload Logger.