docs: add CONTRIBUTING.md

Author: datlechin

CONTRIBUTING.md

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+# Contributing
+
+## Setup
+
+You'll need macOS 14.0+, Xcode 15+, and [Git LFS](https://git-lfs.github.com/) (static libs in `Libs/` are LFS-tracked). Install [SwiftLint](https://github.com/realm/SwiftLint) and [SwiftFormat](https://github.com/nicklockwood/SwiftFormat) too.
+
+```bash
+git clone https://github.com/<your-username>/TablePro.git
+cd TablePro
+git lfs pull
+```
+
+Build with the `-skipPackagePluginValidation` flag (needed for the SwiftLint plugin in CodeEditSourceEditor):
+
+```bash
+xcodebuild -project TablePro.xcodeproj -scheme TablePro -configuration Debug build -skipPackagePluginValidation
+```
+
+Run tests:
+
+```bash
+xcodebuild -project TablePro.xcodeproj -scheme TablePro test -skipPackagePluginValidation
+```
+
+## Code Style
+
+`.swiftlint.yml` and `.swiftformat` are the source of truth. The short version:
+
+- 4-space indentation, 120-char line length target
+- Explicit access control (`private`, `internal`, `public`)
+- No force unwraps or force casts. Use `guard let`, `if let`, `as?`
+- `String(localized:)` for user-facing strings. SwiftUI view literals auto-localize
+- OSLog only, no `print()`
+
+Run both before committing:
+
+```bash
+swiftlint lint --strict
+swiftformat .
+```
+
+## Commits
+
+[Conventional Commits](https://www.conventionalcommits.org/), single line, no body.
+
+```
+feat: add CSV export for query results
+fix: prevent crash on empty query result
+docs: update keyboard shortcuts page
+```
+
+## Branch Naming
+
+Branch off `main`:
+
+- `feat/add-cassandra-support`
+- `fix/query-editor-crash`
+- `docs/update-keyboard-shortcuts`
+
+## Pull Requests
+
+One change per PR. Make sure tests pass and lint is clean. Link related issues.
+
+Before opening, check:
+
+- [ ] Tests added or updated
+- [ ] `CHANGELOG.md` updated under `[Unreleased]` (skip for unreleased-only fixes)
+- [ ] Docs updated in `docs/` and `docs/vi/` if the change affects user-facing behavior
+- [ ] User-facing strings localized
+- [ ] No SwiftLint/SwiftFormat violations
+
+## Project Layout
+
+```
+TablePro/              # App source (Core/, Views/, Models/, ViewModels/, etc.)
+Plugins/               # Database driver .tableplugin bundles
+  TableProPluginKit/   # Shared plugin framework
+  MySQLDriverPlugin/   # MySQL/MariaDB
+  PostgreSQLDriverPlugin/
+  SQLiteDriverPlugin/
+  ...
+Libs/                  # Pre-built static libraries (Git LFS)
+TableProTests/         # Tests
+docs/                  # Mintlify docs site
+scripts/               # Build and release scripts
+```
+
+## Adding a Database Driver
+
+Drivers are `.tableplugin` bundles loaded at runtime. Create a new bundle under `Plugins/`, implement `DriverPlugin` + `PluginDatabaseDriver` from `TableProPluginKit`, and add the target to the Xcode project. Details in `docs/development/plugin-system/`.
+
+## Reporting Bugs
+
+Open a [GitHub issue](https://github.com/datlechin/TablePro/issues) with your macOS version, TablePro version, and reproduction steps. For database-specific bugs, include the database type and version.
+
+## CLA
+
+You'll need to sign the Contributor License Agreement on your first PR. The CLA bot will walk you through it. One-time thing.
+
+## License
+
+Contributions are licensed under [AGPLv3](LICENSE).