Document the release process

docs/releasing.md

@@ -0,0 +1,16 @@
+# Release process
+
+1. `git checkout master && git pull`
+2. `git checkout -b <some-branch-name>`
+3. Update the `version` field in `Cargo.toml`.
+5. Update `CHANGELOG.md`.
+6. Commit `Cargo.toml` and `CHANGELOG.md`. The commit message must start
+   with `release:`.
+7. Push the branch and create a PR.
+8. Merge the PR in "Rebase and merge" mode (to preserve the commit subject).
+
+After merging, the new release will automatically be created on
+<https://crates.io>. A git tag will also be created automatically.
+
+See <https://crates.io/crates/auto-release> for more details of how the
+release process is implemented.