feat(docs): add a contribution guide

Author: ocReaper

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing Guide
+
+Thank you for your interest in contributing! This repository hosts versioned PHP Docker images with optional NVM and Node.js support.
+
+## 📋 Contribution Checklist
+
+Before opening a Pull Request, please make sure that:
+
+- You are modifying or adding files in the correct version folder (e.g. `8.3/Dockerfile`)
+- Your PR **title follows the [Conventional Commits](https://www.conventionalcommits.org/) format** and includes a valid **scope**
+    - ✅ Correct examples:
+        - `feat(8.4): add support for PHP 8.4`
+        - `fix(8.3): correct NVM install path`
+        - `chore(8.3): update nvm version`
+    - ❌ Invalid examples (missing scope or wrong format):
+        - `feat: add PHP 8.4` ← missing scope
+        - `fix php 8.3 image` ← not a conventional commit format
+
+> The `scope` must describe the context of the change, such as the PHP version (`8.0`, `8.3`, etc.) or a general target like `docker` or `ci`.
+
+- Your Dockerfile change builds successfully
+    - The GitHub workflow will attempt to build the image automatically
+    - You can also test locally with:
+      ```sh
+      docker build -t php-custom:8.3 ./8.3
+      ```
+
+## ✅ Best Practices
+
+- **Avoid modifying multiple PHP versions in a single PR** unless necessary
+- Keep version-specific differences minimal and well-documented in `README.md` if needed
+- If adding a new PHP version (e.g. `8.4`), use the closest previous version (e.g. `8.3`) as a starting point
+
+## 🧪 CI Workflow
+
+All PRs trigger a GitHub Actions workflow which will:
+
+- Validate the PR title against Conventional Commits with required scope
+- Build the updated Docker image to ensure it compiles successfully
+
+After a successful squash merge a GitHub Actions workflow will publish the new versions to Docker Hub.
+
+You can find the workflow definitions at [.github/workflows](.github/workflows).
+
+## 🛡️ Security
+
+If you're introducing or updating external binaries (e.g. Node.js, NVM), use official sources and verify integrity (e.g. via checksum or GPG).
+
+If you're unsure about anything, feel free to open a draft PR or discussion.