feat(devmanual): Document PSR-0 autoloader removal

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_27.rst

@@ -30,6 +30,11 @@ Removed APIs
 Back-end changes
 ----------------
 
+Removal of PSR-0 class loader
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Nextcloud 27 no longer loads classes in the deprecated :ref:`PSR-0 naming standard<psr0>`. :ref:`Update the structure to PSR-4<app-psr4-autoloader>` or :ref:`ship a custom autoloader<app-custom-classloader>`.
+
 Added APIs
 ^^^^^^^^^^
 

developer_manual/digging_deeper/classloader.rst

@@ -7,6 +7,8 @@ Classloader
 
 The classloader is provided by Nextcloud and loads all your classes automatically. See :ref:`the composer section<app-composer>` if you want to include and autoload 3rd party libraries.
 
+.. _app-psr4-autoloader:
+
 PSR-4 autoloading
 -----------------