Add document on contributing.

doc/contributing.md

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+# Releasing
+
+These are the items we need to do to release:
+  - Update `CHANGELOG.md`.
+  - Update `version` in `\[workspace.package\]` in `Cargo.toml`.
+  - Commit above changes.
+  - Tag previous changeset with the version tag.
+  - Push changesets and tags.
+
+## Update `CHANGELOG.md`
+
+To update `CHANGELOG.md`, we need to first add missing items, and then change
+the headings to reflect the new release.
+
+### Add Missing Items
+
+The first thing to do when updating the `CHANGELOG.md` is to ensure it is complete.
+To do this, look at the `\[Unreleased\]` link at the bottom of the file in your
+browser and fill in the missing items in the `\[Unreleaed\]` section. Make sure
+you add links to issues when appropriate.
+
+Generally, only add things to `CHANGELOG.md` that you think users would care
+about. There shouldn't be an entry for every changeset.
+
+It's a good idea to commit these changes in a separate changeset.
+
+### Change Headings
+
+The second thing to do is to update the headers in `CHANGELOG.md`. This is
+mechanical. In the future, we will do this using a tool.
+
+These are the steps to take:
+  - Go to the bottom of the file and duplicate the `\[unreleased\]` line. In the
+    bottom copy, change "`unreleased`" to the new version number and change
+    "`HEAD`" to the new release's tag.
+  - In the preceding line (for `\[unreleased\]`, change the "from" tag before
+    "`...HEAD`" to the new release's tag.
+  - Go back to the top of the file, duplicate the `\[Unreleased\]` header, and
+    put an empty line between the two duplicate headers.
+  - Change the second duplicate header to contain the version number and the
+    date, like so:
+    > ## \[10.3.1\] - 2028-06-26
+
+## Update `version` in `\[workspace.package\]` in `Cargo.toml`.
+
+This is a mechanical change. In the future, we will do this using a tool.
+
+Edit `Cargo.toml`, find `version` in `\[workspace.package\]`, and change it to
+the new version.
+
+## Commit Above Changes
+
+This commit should only include the mechanical changes to `CHANGELOG.md` and
+`Cargo.toml`. If you're using `stg`, make sure you commit all of these
+changesets using `stg commit -a`. The message should just be `Version XXX`.
+
+## Tag
+
+Tag the last committed changeset with the release's tag. This will be something
+like "`v1.2.3`". ```bash
+git tag v1.2.3
+```
+
+## Push Changesets and Tags
+
+Ensure that you push, and that you push the tags along with the changesets. We
+can do that atomically like this: ```bash
+git push --atomic origin main v1.2.3
+```