Add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing
+
+GRM is still in very early development. I started GRM mainly to scratch my own
+itches (and am heavily dogfooding it). If you have a new use case for GRM, go
+for it!
+
+The branching strategy is a simplified
+[git-flow](https://nvie.com/posts/a-successful-git-branching-model/).
+
+* `master` is the "production" branch. Each commit is a new release.
+* `develop` is the branch where new stuff is coming in.
+* feature branches branch off of `develop` and merge back into it.
+
+So to contribute, just fork the repo and create a pull request against
+`develop`. If you plan bigger changes, please consider opening an issue first,
+so we can discuss it.
+
+If you want, add yourself to the `CONTRIBUTORS` file in your pull request.
+
+## Code formatting
+
+For Rust, just use `cargo fmt`. For Python, use
+[black](https://github.com/psf/black). I'd rather not spend any effort in
+configuring the formatters (not possible for black anyway).
+
+## Tooling
+
+GRM uses [`just`](https://github.com/casey/just) as a command runner. See
+[here](https://github.com/casey/just#installation) for installation
+instructions (it's most likely just a simple `cargo install just`).
+
+## Testing
+
+There are two distinct test suites: One for unit test (`just test-unit`) and
+integration tests (`just test-integration`) that is part of the rust crate, and
+a separate e2e test suite in python (`just test-e2e`).
+
+To run all tests, run `just test`.
+
+When contributing, consider whether it makes sense to add tests that to prevent
+regressions in the future. When fixing bugs, it makes sense to add tests that
+expose the wrong behaviour beforehand.
+
+## Documentation
+
+The documentation lives in `docs` and uses
+[mdBook](https://github.com/rust-lang/mdBook).  Please document new user-facing
+features here!
+

docs/src/SUMMARY.md

@@ -5,3 +5,4 @@
 - [Repository trees](./repos.md)
 - [Git Worktrees](./worktrees.md)
 - [FAQ](./faq.md)
+- [Contributing](./contributing.md)

docs/src/contributing.md

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+../../CONTRIBUTING.md