Update README

Author: schmengler

README.md

@@ -300,6 +300,17 @@ public function testSomethingWithMultipleProducts()
 
 ```
 
+### Config Fixtures
+
+With the config fixture you can set a configuration value globally, i.e. it will ensure that it is not only set in the default scope but also in all store scopes:
+```
+ConfigFixture::setGlobal('general/store_information/name', 'Ye Olde Wizard Shop');
+```
+
+It uses `MutableScopeConfigInterface`, so the configuration is not persisted in the database. Use `@magentoAppIsolation enabled` in your test to make sure that changes are reverted in subsequent tests.
+
+You can also set configuration values explicitly for stores with `ConfigFixture::setForStore()` 
+
 ## Credits
 
 - [Fabian Schmengler][link-author]