Document suggestions for a jj development environment

I worte up some things I wish I knew when I wrote my first PR.

docs/contributing.md

@@ -1,9 +1,11 @@
 # How to Contribute
 
+## Policies
+
 We'd love to accept your patches and contributions to this project. There are
 just a few small guidelines you need to follow.
 
-## Contributor License Agreement
+### Contributor License Agreement
 
 Contributions to this project must be accompanied by a Contributor License
 Agreement. You (or your employer) retain the copyright to your contribution;
@@ -15,14 +17,65 @@ You generally only need to submit a CLA once, so if you've already submitted one
 (even if it was for a different project), you probably don't need to do it
 again.
 
-## Code reviews
+### Code reviews
 
 All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose. Consult
 [GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
 information on using pull requests.
 
-## Community Guidelines
+### Community Guidelines
 
 This project follows [Google's Open Source Community
 Guidelines](https://opensource.google/conduct/).
+
+## Setting up a development environment
+
+To develop `jj`, the mandatory steps are simply to [install
+Rust](https://www.rust-lang.org/tools/install), clone the repository,
+and use `cargo build`, `cargo fmt`, `cargo test`, etc.  If you are
+preparing a PR, there are some additional recommended steps.
+
+### Summary
+
+One-time setup:
+
+    rustup toolchain add nightly  # If this is not your default
+    rustup toolchain add 1.60
+    cargo install cargo-insta
+    cargo install cargo-watch
+
+During development (adapt according to your preference):
+
+    cargo watch --ignore '.jj/**' -s 'cargo +nightly fmt && cargo clippy && cargo +1.60 check'
+    cargo insta test  # Occasionally
+
+WARNING: Build artifacts from debug builds and especially from repeated
+invocations of `cargo test` can quickly take up 10s of GB of disk space.
+Cargo will happily use up your entire hard drive. If this happens, run
+`cargo clean`.
+
+### Explanation
+
+These are ordered roughly in order of decreasing importance.
+
+1. Nearly any change to `jj`'s interface will require writing or updating
+golden tests that use the [`insta`](https://insta.rs/) crate.  To make
+this convenient, install the `cargo-insta` binary. Use `cargo insta tests`
+to run tests, and `cargo insta review` to update the golden tests.
+
+2. Github CI checks require that the code is formatted with the *nightly*
+version of `rustfmt`. To do this on your computer, install the nightly
+toolchain and use `cargo +nightly fmt`.
+
+3. Your code will be rejected if it cannot be compiled with the
+minimal supported version of Rust. This version is listed as
+`rust-version` in [`Cargo.toml`](../Cargo.toml); it is 1.60 as
+of this writing.
+
+4. Your code needs to pass `cargo clippy`. You can also use `cargo
++nightly clippy` if you wish to see more warnings.
+
+5. You may also want to install and use `cargo-watch`. In this case,
+you should exclude `.jj`. directory from the filesystem watcher, as
+it gets updated on every `jj log`.