Explain manual installation

Author: ruudk

README.md

@@ -10,8 +10,22 @@
 
 ## Installation
 
-```command
-$ composer require --dev ticketswap/phpstan-error-formatter
+To use this extension, require it in [Composer](https://getcomposer.org/):
+
+```bash
+composer require --dev ticketswap/phpstan-error-formatter
+```
+
+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`, include extension.neon in your project's PHPStan config:
+
+```neon
+includes:
+    - vendor/ticketswap/phpstan-error-formatter/extension.neon
 ```
 
 ## Usage