Add docs

Author: jissereitsma

README.md

@@ -23,3 +23,23 @@ Or generate unit tests:
 ```bash
 bin/magento yireo:test:generate Yireo_Example --type=unit
 ```
+
+### Extending test types
+The Integration Tests can be created by using a custom generator. For instance, there is a `ConfigTestGenerator` which allows for a
+specific config test case to be created, as long as the PHP class name matches with `*\Config\Config`.
+
+```xml
+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
+    <type name="Yireo\TestGenerator\Generator\IntegrationTestGenerator">
+        <arguments>
+            <argument name="testGenerators" xsi:type="array">
+                <item name="custom" xsi:type="object">YireoTraining\ExampleTestGenerator\Generator\IntegrationTest\CustomTestGenerator</item>
+            </argument>
+        </arguments>
+    </type>
+</config>
+```
+
+This generator class needs to implement `\Yireo\TestGenerator\Generator\IntegrationTest\TestGeneratorInterface`. See the
+`\Yireo\TestGenerator\Generator\IntegrationTest\ConfigTestGenerator` for a full example.