docs: add [development] section to CHANGELOG, document in CONTRIBUTING

Record ongoing changes under [development] in CHANGELOG.md. Update the
release workflow to rename [development] to the versioned section.

CHANGELOG.md

@@ -5,6 +5,24 @@ All notable changes to **volumen** are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [development]
+
+### Added
+
+- `<pubDate>` in RSS feed items, derived from the post date.
+- `<lastmod>` in sitemap entries for SEO.
+- In-memory rate limiting on the login endpoint: 10 attempts per 60 seconds per IP.
+
+### Changed
+
+- Split `lib/volumen/server.rb` into `api_routes.rb` and `admin_routes.rb` modules,
+  keeping the server class focused on configuration and shared helpers.
+
+### Fixed
+
+- `admin.session_ttl` was defined but never read; it now controls
+  `Rack::Session::Cookie` expiration via `expire_after`.
+
 ## [0.1.0] — 2026-06-20
 
 First public release: a small, file-based Markdown blog engine in CRuby. Posts