Document the drupal_root parameter

Fixes #46

Author: mglaman

README.md

@@ -75,6 +75,24 @@ includes:
 
 ## Adapting to your project
 
+### Specifying your Drupal project's root
+
+By default, the PHPStan Drupal extension will try to determine your Drupal project's root directory based on the working
+directory that PHPStan is checking. If this is not working properly, you can explicitly define the Drupal project's root
+directory using the `drupal_root` parameter.
+
+```
+drupal:
+    drupal_root: /path/to/drupal
+```
+
+You can also use container parameters. For instance you can always set it to the current working directory.
+
+```
+drupal:
+    drupal_root: %currentWorkingDirectory%
+```
+
 ### Entity storage mappings.
 
 The `EntityTypeManagerGetStorageDynamicReturnTypeExtension` service helps map dynamic return types. This inspects the