docs: Add release instructions to DEVELOPMENT.md (#1070)

Co-authored-by: doug-q <[email protected]>

DEVELOPMENT.md

@@ -136,3 +136,50 @@ We accept the following contribution types:
 - ci: CI related changes. These changes are not published in the changelog.
 - chore: Updating build tasks, package manager configs, etc. These changes are not published in the changelog.
 - revert: Reverting previous commits.
+
+## :shipit: Releasing new versions
+
+We use automation to bump the version number and generate changelog entries
+based on the [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) labels. Release PRs are created automatically
+for each package when new changes are merged into the `main` branch. Once the PR is
+approved by someone in the [release team](.github/CODEOWNERS) and is merged, the new package
+is published on PyPI or crates.io as appropriate.
+
+The changelog can be manually edited before merging the release PR. Note however
+that modifying the diff before other changes are merged will cause the
+automation to close the release PR and create a new one to avoid conflicts.
+
+### Rust crate release
+
+Rust releases are managed by `release-plz`. This tool will automatically detect
+breaking changes even when they are not marked as such in the commit message,
+and bump the version accordingly.
+
+To modify the version being released, update the `Cargo.toml`,
+CHANGELOG.md, PR name, and PR description in the release PR with the desired version. You may also have to update the dates.
+Rust pre-release versions should be formatted as `0.1.0-alpha.1` (or `-beta`, or `-rc`).
+
+### Python package release
+
+Python releases are managed by `release-please`. This tool always bumps the
+minor version (or the pre-release version if the previous version was a
+pre-release).
+
+To override the version getting released, you must merge a PR to `main` containing
+`Release-As: 0.1.0` in the description.
+Python pre-release versions should be formatted as `0.1.0a1` (or `b1`, `rc1`).
+
+### Patch releases
+
+Sometimes we need to release a patch version to fix a critical bug, but we don't want
+to include all the changes that have been merged into the main branch. In this case,
+you can create a new branch from the latest release tag and cherry-pick the commits
+you want to include in the patch release.
+
+You will need to modify the version and changelog manually in this case. Check
+the existing release PRs for examples on how to do this. Once the branch is
+ready, create a [github release](https://github.com/CQCL/hugr/releases/new).
+The tag should follow the format used in the previous releases, e.g. `hugr-py-v0.1.1`.
+
+For rust crates, you will need someone from the release team to manually
+publish the new version to crates.io by running `cargo release`.