diff --git a/doc/Running Unit Tests.md b/doc/Running Unit Tests.md
index 95078258..1a5770bc 100644
--- a/doc/Running Unit Tests.md	
+++ b/doc/Running Unit Tests.md	
@@ -43,6 +43,14 @@ Example for Debian and Ubuntu:
 $ sudo apt install phpunit php-gd php-sqlite3 php-xdebug
 ```
 
+Because the unit tests depend on this, you also need to install the optional. Otherwise they won't run:
+```console
+composer require google/cloud-storage
+```
+
+If you do this and want to develop further, please go into `.gitignore` and adjust it to ignore the whole
+vendor directory. Otherwise your `git status` will be filled with lot's of unrelated PHP files.
+
 To run the tests, change into the `tst` directory and run phpunit:
 
 ```console