Add missing reflinks to changelog

See the guide: https://keepachangelog.com/en/1.0.0/

Author: glensc

CHANGELOG.md

@@ -36,6 +36,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - GH Actions: run the tests against all supported PHP versions [#55](https://github.com/php-parallel-lint/PHP-Parallel-Lint/pull/55) from [@jrfnl](https://github.com/jrfnl).
 - GH Actions: report CS violations in the PR [#55](https://github.com/php-parallel-lint/PHP-Parallel-Lint/pull/55) from [@jrfnl](https://github.com/jrfnl).
 
+[Unreleased]: https://github.com/php-parallel-lint/PHP-Parallel-Lint/compare/v1.2.0...HEAD
+
 ## [1.2.0] - 2020-04-04
 
 ### Added
@@ -56,3 +58,5 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Added info about installing like not a dependency.
 - Cleaned readme - new organization from previous package.
 - Added checklist for new version from [@szepeviktor](https://github.com/szepeviktor).
+
+[1.2.0]: https://github.com/php-parallel-lint/PHP-Parallel-Lint/compare/v1.1.0...v1.2.0