Add contributing instructions to the readme.

Author: SiegeLordEx

Readme.md

@@ -54,3 +54,36 @@ A somewhat involved 2D example (see `example1.rs` in the `examples` directory):
 ```
 cargo build
 ```
+
+## Contributing
+
+### Style
+
+Please run `cargo fmt` before sending pull requests.
+
+### Output tests
+
+The CI verifies that the PNG outputs do not unexpectedly change for existing
+examples. This is done by checking out the repository before your PR and after
+the PR and comparing the outputs. If you changed some examples deliberately,
+you can indicate this in the PR description by adding a line like:
+
+```
+CHANGED_OUTPUTS=image1.png,image2.png
+```
+
+where `image1` etc is derived from the string you pass to the `c.show(&mut fg,
+"image1");` line in the example. To run the tests manually you run these two
+commands (requires [uv](https://github.com/astral-sh/uv) to be installed):
+
+```bash
+source setup_venv.sh  # Do this once
+. venv/bin/activate   # Do this if you already set up venv
+./cargo_util.py --make_golden_outputs  # On the base commit (typically master)
+./cargo_util.py --test_outputs --ignore_new_outputs --changed_outputs=image1.png,image2.png  # With your changes applied
+```
+
+We don't check in the golden outputs because gnuplot does not guarantee
+cross-platform pixel-perfect outputs, so the outputs end up being specific to
+the platform they're generated on. Thus, we only compare two commits instead on
+the same platform (i.e. your local machine, or the CI runner).