Added documentation

docs/en/plugins/php_unit.md

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+Runs PHPUnit tests against your build.
+## Configuration
+### Options
+Has two modes:
+
+#### phpunit.xml Configuration File
+Its activated if you have phpunit.xml file in your build path, `tests/` subfolder, or you specify it as a parameter:
+* **config** - Optional - Path to a PHP Unit XML configuration file.
+* **run_from** - Optional - When running PHPUnit with an XML config, the command is run from this directory
+* **coverage** - Optional - Value for the `--coverage-html` command line flag.
+* **path** - Optional - In cases where tests files are in a sub path of the /tests path, allows this path to be set in the config.
+
+#### Running Tests By Specifying Directory
+* **directory** - Optional - The directory (or array of dirs) to run PHPUnit on
+
+Both modes accept:
+* **args** - Optional - Command line args (in string format) to pass to PHP Unit
+
+### Examples
+Specify config file and test directory:
+```yml
+test:
+    php_unit:
+        config:
+            - "path/to/phpunit.xml"
+        path: "app/tests/"
+```
+
+## Troubleshooting
+If standard logging of PHPCI is not enough, to get standard output from any command, including PHPUnit, edit `BaseCommandExecutor::executeCommand()` to see what exactly is wrong
+* Run `composer update` in phpunit plugin directory of PHPCI to get all of its dependencies
+* If phpunit is inside of the project's composer.json, it might interfere with PHPCI's phpunit installation
+* Make sure you have XDebug installed.`The Xdebug extension is not loaded. No code coverage will be generated.`
+Otherwise test report parsing in `TapParser` will fail, wanting coverage report as well `Invalid TAP string, number of tests does not match specified test count.`