Install composer phpunit v9 for development testing

vendor/sebastian/code-unit/README.md

@@ -0,0 +1,17 @@
+# sebastian/code-unit
+
+Collection of value objects that represent the PHP code units.
+
+## Installation
+
+You can add this library as a local, per-project dependency to your project using [Composer](https://getcomposer.org/):
+
+```
+composer require sebastian/code-unit
+```
+
+If you only need this library during development, for instance to run your project's test suite, then you should add it as a development-time dependency:
+
+```
+composer require --dev sebastian/code-unit
+```