Docs: add a note on adding use on theme.php

doc/themes.md

@@ -167,6 +167,8 @@ The content of this file should be something like
 
     <?php
     /* meta informations for the theme, see below */
+    use Friendica\App;
+    
     function duepuntozero_lr_init(App $a) {
         $a-> theme_info = array(
             'extends' => 'duepuntozero'.
@@ -246,6 +248,12 @@ These three pieces of information should be listed.
 If the original author is no longer working on the theme, but a maintainer has taken over, the maintainer should be listed as well.
 The information from the theme header will be displayed in the admin panel.
 
+The first thing in file is to import the `App` class from `\Friendica\` namespace.
+
+    use Friendica\App;
+
+This will make our job a little easier, as we don't have to specify the full name every time we need to use the `App` class.
+
 The next crucial part of the theme.php file is a definition of an init function.
 The name of the function is <theme-name>_init.
 So in the case of quattro it is