Add changelog command to docs

Human-readable code changes included in each release are often kept in a
"changelog," which is a text file named CHANGELOG.

Manually-generated changelogs often have errors and omissions. On the
other hand, auto-generated changelogs often require extra dependencies,
do not include commit-level info, and do not annotate Git tags.

This project does not currently provide a CHANGELOG.md, but does provide
annotated and signed Git tags with informative tag messages. These tags
can be used to auto-generate a changelog.

This commit will add a changelog command to the contributing docs.

docs/contributing.md

@@ -266,3 +266,18 @@ Change the port numbers to run multiple containers simultaneously (`-p 81:80`).
         -   Omit the leading `v` (use `1.0.0` instead of `v1.0.0`)
         -   Example: `git tag -a -s 1.0.0`
     -   Push the tag. GitHub Actions will build and push the Python package and Docker images.
+-   **To create a changelog:**
+
+    ```sh
+    printf '# Changelog\n\n' >CHANGELOG.md
+
+    GIT_LOG_FORMAT='## %(subject) - %(taggerdate:short)
+
+    %(contents:body)
+    Tagger: %(taggername) %(taggeremail)
+    Date: %(taggerdate:iso)
+
+    %(contents:signature)'
+
+    git tag -l --sort=-taggerdate:iso --format="$GIT_LOG_FORMAT" >>CHANGELOG.md
+    ```