Add docs for app/Common.php

Author: MGatner

user_guide_src/source/extending/common.rst

@@ -0,0 +1,13 @@
+************************
+Replace Common Functions
+************************
+
+There a qutie a few functions necessary to CodeIgniter that need to load for use in the core classes and thus cannot
+be placed into a helper. While most users will never have any need to do this, but the option to replace these functions
+does exist for those who would like to significantly alter the CodeIgniter core. In the ``App\`` directory there is a
+file ``Common.php``, and any functions defined in there will take precedence over the versions found in
+``system/Common.php``. This is also an opportunity to create globally-available functions you intend to use throughout
+the framework.
+
+.. note:: Messing with a core system class has a lot of implications, so make sure you know what you are doing before
+    attempting it.