readme: added submodule update precision

README.md

@@ -165,7 +165,12 @@ once inside the php container in /var/www/html you can use drush as usual
 
 ```
 git pull figli master
+# then update all submodules
 git submodule update --recursive --checkout
+# or only one
+git submodule update app/
+# or
+git submodule update api/
 make api_maj_config
 ```