[TASK] Update configuration on how to with use phpstan/extension-installer

This updates the configuration part on how to use this extension with
or without phpstan/extension-installer.

Author: Manuel Selbach

README.md

@@ -3,12 +3,27 @@
 TYPO3 CMS class reflection extension for PHPStan & framework-specific rules
 
 ## Configuration
-Put this into your phpstan.neon config:
+
+To use this extension, require it in [Composer](https://getcomposer.org/):
+
+```
+composer require --dev saschaegerer/phpstan-typo3
+```
+
+If you also install [phpstan/extension-installer](https://github.com/phpstan/extension-installer) then you're all set!
+
+<details>
+  <summary>Manual installation</summary>
+
+If you don't want to use `phpstan/extension-installer`, put this into your phpstan.neon config:
 
 ```
 includes:
     - vendor/saschaegerer/phpstan-typo3/extension.neon
 ```
+</details>
+
+## Additional Configuration
 
 If you do use constants of TYPO3 core you may have to
 bootstrap TYPO3 first. This can be done by using the