Docs: add contributing md.

.github/CODEOWNERS

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+# Codeowners
+*       @mir-evgenii

CONTRIBUTING.md

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+# Contributing to liboqs-php
+
+A short guide on how to get the project, make changes, and submit a Pull Request.
+For installing liboqs and PHP requirements, see [README.md](./README.md).
+
+## 1. Fork and clone
+
+1. Click **Fork** on `https://github.com/mir-evgenii/liboqs-php`.
+2. Clone your fork:
+```bash
+git clone https://github.com/<your-username>/liboqs-php.git
+cd liboqs-php
+git remote add upstream https://github.com/mir-evgenii/liboqs-php.git
+```
+
+3. Install dependencies:
+```bash
+composer install
+```
+
+## 2. Create a branch
+
+Changes are made on a separate branch, never directly on `main`:
+
+```bash
+git checkout main
+git pull upstream main
+git checkout -b fix/short-description
+```
+
+Use the `fix/`, `feature/`, or `docs/` prefix depending on the change.
+
+If you're picking up an existing GitHub issue, leave a quick comment on it saying
+you're working on it. This helps avoid two people submitting a PR for the same issue.
+
+## 3. Make your changes
+
+Keep the PR focused on a single task. Follow the coding style already used in `src/`.
+If you're changing anything FFI-related (`liboqs_ffi.h`, `resolveLibraryPath()`), note
+in the PR which liboqs version and OS you tested it on.
+
+## 4. Run the tests
+
+```bash
+vendor/bin/phpunit
+```
+
+All tests must pass locally before submitting the PR. If you're adding new
+functionality, add a test for it.
+
+## 5. Commit
+
+```bash
+git add .
+git commit -m "fix: short description of the change"
+```
+
+## 6. Update your branch before opening the PR
+
+```bash
+git fetch upstream
+git rebase upstream/main
+```
+
+If there are conflicts, resolve them and run `git rebase --continue`, then run the
+tests again.
+
+## 7. Open the PR
+
+```bash
+git push origin fix/short-description
+```
+
+Open a PR from your branch into `main` of `mir-evgenii/liboqs-php`. In the
+description, include:
+
+- **Problem** — what was wrong (if there's an issue: `Closes #<number>`).
+- **Solution** — what changed and why.
+- **Testing** — which tests passed, on which OS/version you verified it.
+Example:
+
+```markdown
+## Problem
+resolveLibraryPath() doesn't find liboqs on Apple Silicon (Homebrew).
+
+## Solution
+Added /opt/homebrew/lib as a fallback path for macOS.
+
+## Testing
+Verified on macOS 14 / PHP 8.2 / liboqs 0.10.0. `vendor/bin/phpunit` passes.
+
+Closes #12
+```
+
+After submitting, wait for CI to pass and respond to reviewer comments if any — just
+push new commits to the same branch.