Add changelog in preparation for stable release [skip CI]

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file. For previous changes, refer to the [CHANGELOG-0.x.md](https://github.com/facile-it/paraunit/blob/0.12.x/CHANGELOG-0.x.md) document.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+## Unreleased
+
+## [1.0.0] - TBA
+First stable release. The following changes are in comparison to the previous, unstable release (0.12.3)
+
+### Breaking changes
+ * Drop support for older packages: [#134](https://github.com/facile-it/paraunit/pull/134)
+   * `phpunit/phpunit` 6 
+   * `phpunit/php-code-coverage` < 6
+   * All Symfony components < 3.4
+ * Scalar and return types added everywhere possible
+
+### Added
+ * Add support for `phpunit/phpunit` 8 and `phpunit/php-code-coverage` 7 [#133](https://github.com/facile-it/paraunit/pull/133)
+ * Add explicit requirement for `ext-dom` and `ext-json` [#134](https://github.com/facile-it/paraunit/pull/134)
+
+### Fixed
+ * Do not set values on PHPUnit options that do not expect values [#127](https://github.com/facile-it/paraunit/pull/127), thanks @fullbl
+
+### Changed
+ * Update PHPStan to 0.11 [#128](https://github.com/facile-it/paraunit/pull/128)
+ * Update coding standard to 0.3 [#131](https://github.com/facile-it/paraunit/pull/131)
+ * Disable Scrutinizer [#132](https://github.com/facile-it/paraunit/pull/132)