Added documentation

docs/en/plugins/codeception.md

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+A simple plugin that allows you to run [Codeception](http://codeception.com/) tests.
+
+### Configuration Options:
+
+* **config** - Required - Can be either a single string pointing to a Codeception configuration file, or an array of configuration file paths. By default this is called `codeception.yml` and will be in the root of your project.
+
+* **args** - Optional - The string of arguments to be passed to the run command. **Important**, due to the assumption made on line [132](https://github.com/Block8/PHPCI/blob/master/PHPCI/Plugin/Codeception.php#L132) regarding the value of `--xml` being the next argument which will not be correct if the user provides arguments using this config param, you must specify `report.xml` before any user input arguments to satisfy the report processing on line [146](https://github.com/Block8/PHPCI/blob/master/PHPCI/Plugin/Codeception.php#L146)
+
+* **path** - Optional - The path from the root of your project to the root of the codeception _output directory
+
+##### Default values
+
+- config
+ - `codeception.yml` if it exists in the root of the project
+ - `codeception.dist.yml` if it exists in the root of the project
+ - null if no option provided and the above two fail, this will cause an Exception to be thrown on execution
+
+- args
+ - Empty string
+
+- path
+ - `tests/_output/`
+
+##### Example on running codeception with default settings (when tests are in tests/ directory):
+
+```
+  codeception:
+    config: "codeception.yml"
+    path: "tests/"
+```
+
+##### Example usage against the Yii2 framework
+
+```
+codeception:
+        allow_failures: false
+        config: "tests/codeception.yml"
+        path: "tests/codeception/_output/"
+        args: "report.xml --no-ansi --coverage-html"
+```
+
+The path value will need to be changed if you have your tests directory somewhere other than in the root of the project.