docs: add release checklist (#972)

RELEASE_CHECKLIST.md

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+# Release checklist
+
+This checklist is meant to be used as a guide for the `crates.io` release process.
+
+Releases are always made in lockstep, meaning that all crates in the repository
+are released with the same version number, regardless of whether they have
+changed or not.
+
+## Requirements
+
+- [cargo-release](https://github.com/crate-ci/cargo-release): `cargo install cargo-release`
+- [git-cliff](https://github.com/orhun/git-cliff) (patched with [orhun/git-cliff#711](https://github.com/orhun/git-cliff/pull/711)): `cargo install --git https://github.com/DaniPopes/git-cliff.git --branch fix-include-paths git-cliff`
+
+## Steps
+
+- [ ] Make sure you're on the `main` branch.
+- [ ] (optional) Dry run `cargo-release`: `cargo release <version>`
+- [ ] Run `cargo-release`: `PUBLISH_GRACE_SLEEP=10 cargo release <version> --rate-limit.existing-packages 50 [--no-verify]`
+  - Ignore these warnings:
+    - `warning: updating <crate> to <version> despite no changes made since...`
+    - `git-cliff` warning `there is already a tag (<tag>) for ...`
+  - [ ] If a failure happened:
+    - [ ] You should have an unpushed commit. After the issue is fixed, retry the release process with `--no-push` and squash the commits together.
+    - [ ] If some crates were published before the error, AFAICT you must manually `--exclude <crate>` each already-published crate.
+    - [ ] Verify that the commit is correct, and push to the repository with `git push --tags`.
+- [ ] Create a new GitHub release with the automatically generated changelog and with the name set to `<repo> v<X.Y.Z>`