CHANGELOG: bring back Unreleased

Apparently, the GitHub Actions workflows from the template (see initial
commit) require `Unreleased` to always be at the top of the changelog.

The failing build failed at command:

    ./gradlew getChangelog --unreleased --no-header --console=plain -q

See field `unreleasedTerm` in class ChangelogPluginExtension

Note, that for some reason Gradle 8.1.1 doesn't realize that changes in
CHANGELOG.md require throwing away configuration cache.  I had to use
flag `--no-configuration-cache` locally to convince it to re-run the
build properly.

CHANGELOG.md

@@ -2,7 +2,9 @@
 
 # Copy Commit Reference Changelog
 
-## 0.1.0-alpha.1
+## Unreleased
+
+## 0.1.0-alpha.1 - 2023-06-25
 ### Added
 - Support for "Copy Commit Reference" action in the following context menus:
   - Tool window "Git":

README.md

@@ -12,8 +12,8 @@ See [LICENSE.txt](LICENSE.txt) for details.
 - [x] Get familiar with the [template documentation][template].
 - [x] Adjust the [pluginGroup](./gradle.properties), [plugin ID](./src/main/resources/META-INF/plugin.xml) and [sources package](./src/main/kotlin).
 - [x] Adjust the plugin description in `README` (see [Tips][docs:plugin-description])
-- [ ] Review the [Legal Agreements](https://plugins.jetbrains.com/docs/marketplace/legal-agreements.html?from=IJPluginTemplate).
-- [ ] [Publish a plugin manually](https://plugins.jetbrains.com/docs/intellij/publishing-plugin.html?from=IJPluginTemplate) for the first time.
+- [x] Review the [Legal Agreements](https://plugins.jetbrains.com/docs/marketplace/legal-agreements.html?from=IJPluginTemplate).
+- [x] [Publish a plugin manually](https://plugins.jetbrains.com/docs/intellij/publishing-plugin.html?from=IJPluginTemplate) for the first time.
 - [ ] Set the `PLUGIN_ID` in the above README badges.
 - [ ] Set the [Plugin Signing](https://plugins.jetbrains.com/docs/intellij/plugin-signing.html?from=IJPluginTemplate) related [secrets](https://github.com/JetBrains/intellij-platform-plugin-template#environment-variables).
 - [ ] Set the [Deployment Token](https://plugins.jetbrains.com/docs/marketplace/plugin-upload.html?from=IJPluginTemplate).