Merge pull request #6042 from kenjis/docs-add-upgrade-procedure-v420

docs: add upgrade procedure example for v4.2.0

Author: kenjis

user_guide_src/source/installation/upgrade_420.rst

@@ -24,6 +24,15 @@ The following files received significant changes and
 * ``public/index.php``
 * ``spark``
 
+.. important:: If you don't update the above two files, CodeIgniter will not work at all
+    after running ``composer update``.
+
+    The upgrade procedure, for example, is as follows::
+
+        > composer update
+        > cp vendor/codeigniter4/framework/public/index.php public/index.php
+        > cp vendor/codeigniter4/framework/spark .
+
 Config/Constants.php
 ====================