changelog: Improve

Remove 'unrelease' section for as long as there are no unreleased
entries. Add missing GitHub issue link.

CHANGELOG.md

@@ -5,8 +5,6 @@ All notable changes to this project will be 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).
 
-## [Unreleased]
-
 ## [0.3.0] - 2022-11-07
 
 ### Fixed
@@ -18,7 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Events / annotations can be queried as metrics. This allows to visualize for
-  example today's sunrise/sunset time on dashboards.
+  example today's sunrise/sunset time on dashboards. ([#20](https://github.com/fetzerch/grafana-sunandmoon-datasource/issues/20))
 - Dashboard options such as units are automatically set.
 
 ### Changed