docs: Add CONTRIBUTING.md and LICENSE files to the repository

CONTRIBUTING.md

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+# Contributing to gsplash
+
+Thanks for your interest in contributing. gsplash is a small utility and contributions that keep it that way are most welcome.
+
+## What to work on
+
+Check the issue tracker on [GitHub](https://github.com/sortedcord/gsplash) for open issues. If you want to add something new, open an issue first to discuss it before writing code. This avoids wasted effort if the change isn't a good fit for the project.
+
+Good areas to contribute:
+
+- Bug fixes (see known issues in README or open issues)
+- Broader video format or image format support
+- Wayland compatibility improvements
+- Packaging for other distributions
+
+## Building
+
+You'll need:
+
+- GCC or Clang
+- make, pkg-config
+- SDL2, SDL2\_image
+- ffmpeg (`libavformat`, `libavcodec`, `libswscale`, `libavutil`)
+
+On Arch Linux:
+
+```bash
+sudo pacman -S sdl2 sdl2_image ffmpeg
+```
+
+On Ubuntu/Debian:
+
+```bash
+sudo apt install libsdl2-dev libsdl2-image-dev libavformat-dev libavcodec-dev libswscale-dev libavutil-dev
+```
+
+Then build:
+
+```bash
+make
+```
+
+The binary is output to `build/gsplash`.
+
+## Testing
+
+Before submitting a PR, run the automated test suite:
+
+```bash
+make check
+```
+
+This runs a headless smoke test and the CLI argument validation suite. Both must pass.
+
+For visual changes (render modes, layout), also run the interactive test with a real image:
+
+```bash
+./tests/test_interactive.sh path/to/image.png
+```
+
+If you're adding new CLI arguments or flags, add corresponding cases to `tests/test_cli.sh`.
+
+## Code style
+
+- Match the existing style: 4-space indentation, `snake_case`, braces on their own line.
+- All new functions in `video.c`/`video.h` should have declarations in the header.
+- Avoid adding new dependencies unless there's a strong reason. SDL2 and ffmpeg are already heavy. 
+
+## Submitting a PR
+
+1. Fork the repo and create a branch from `main`.
+2. Keep commits focused — one logical change per commit.
+3. Write a clear PR description explaining what the change does and why.
+4. If the change fixes a bug, reference the issue number.
+5. PRs should target `github.com/sortedcord/gsplash` — this is the canonical public remote.
+
+## Reporting bugs
+
+Open an issue on GitHub with:
+
+- Your OS and desktop environment
+- The gsplash version or commit hash
+- The command you ran
+- The full stderr output (gsplash logs to stderr via `[splash]` prefixed lines)