tree: add CONTRIBUTING.md

This will be helpful for future contributors. I also wanted a place
to write down my thoughts on using AI coding agents responsibly and
respectfully.

Assisted-by: Claude Opus 4.6

CONTRIBUTING.md

@@ -0,0 +1,112 @@
+# Contributing to chunkah
+
+## Getting started
+
+chunkah is written in Rust so you'll need a Rust toolchain set up. It also uses
+[just](https://github.com/casey/just) instead of the more common `make`.
+
+To build chunkah and run unit tests:
+
+```bash
+just check
+```
+
+Your iteration loop as you're hacking on chunkah will likely be mostly this in
+early development.
+
+There is also `just checkall` which runs additional checks/lints. Run this
+before submitting a PR.
+
+## e2e tests
+
+To run the e2e tests, you first need to build the chunkah image locally. This
+can be done using
+
+```bash
+just buildimg --no-chunk
+```
+
+(The `--no-chunk` option skips the self-chunking step for faster builds.)
+
+And then you can run e2e tests with
+
+```bash
+just test
+```
+
+It also supports running just some tests (e.g. `just test reproducible`).
+
+## Support tooling
+
+There's also various development tools available to make it easier to debug/test
+code. Two are documented here, but definitely look at all the scripts in the
+`tools/` directory.
+
+### just split
+
+Helper to split an image using the `Containerfile.splitter` flow. Example usage:
+
+```bash
+just split quay.io/fedora/fedora -v
+```
+
+### just diff
+
+Helper to diff between two images, usually the original image and the result of
+passing it through chunkah. Example usage:
+
+```bash
+just diff quay.io/fedora/fedora-minimal localhost/fedora-minimal
+```
+
+## AI-assisted contributions
+
+This project heavily uses AI for development. This section sets clear
+expectations on using AI for contributions. The guidance below is inspired by
+Oxide's [RFD 576] on using LLMs (the whole doc is definitely worth a read!).
+
+### You own what you submit
+
+However the code was produced, the contributor bears full responsibility for
+it. AI is a tool; the human using it is accountable for the result. **This means
+you MUST understand every line of code you submit** and be able to explain the
+design decisions behind it.
+
+### Self-review is mandatory
+
+**DO NOT request reviews on AI-generated code you did not yourself review
+first**. Check for correctness and clarity. If you wouldn't be comfortable
+defending a line of code in review, don't submit it. Failure to do this
+self-review and catching obvious AI slop during the review process wastes time
+and erodes trust.
+
+### Don't re-generate in response to review
+
+You can use AI to help address review feedback, but it must be intentional and
+targeted to the review areas. **DO NOT regenerate the whole contribution**. The
+interdiff (the diff between the last reviewed patches and your addressing of
+them) must be in line with what reviewers expect given the feedback they gave.
+
+### Disclose AI usage
+
+Disclosure of AI usage is recommended but not mandatory. The best way to
+disclose it is via an `Assisted-by:` trailer in your commit messages to indicate
+which AI tool was used. For example:
+
+```text
+Assisted-by: Claude Opus 4.6
+```
+
+This isn't about shaming or gatekeeping (many/most commits in this repo are
+AI-assisted!). It's about providing useful context to make review easier, and
+to help us learn collectively about where AI assistance works well and where
+it doesn't.
+
+### The AGENTS.md file
+
+The [AGENTS.md](AGENTS.md) file provides detailed guidance to AI coding agents
+working in this repository, including project architecture, coding
+conventions, and build commands. If you use an AI coding agent, make sure it
+has access to this file.
+
+[RFD 576]: https://rfd.shared.oxide.computer/rfd/0576