Add note about tests modifying composer.json

Author: stancl

README.md

@@ -11,3 +11,19 @@ composer require leanadmin/installer
 ```sh
 php artisan lean:setup
 ```
+
+## Development
+
+Run all checks:
+
+```
+./check
+```
+
+Run tests:
+
+```
+phpunit
+```
+
+**Make sure to clean up `composer.json` when running tests that modify it.**