Add Moving from J to Namespace

Moving from J to Namespace

@@ -0,0 +1,16 @@
+Class map in J3:
+
+[https://github.com/joomla/joomla-cms/blob/3.10.12/libraries/classmap.php]
+
+Class map in J5:
+
+[https://github.com/joomla/joomla-cms/blob/5.0-dev/plugins/behaviour/compat/src/classmap/classmap.php]
+
+How to handle Classes that have no new counterparts like:
+
+- Jerror
+JError::raiseError() should be replaced with native exceptions.
+JError::raiseWarning() and JError::raiseNotice should be replaced with calls to application's enqueueMessage() method to display warnings/notices.
+
+More info on backwars compatible issues:
+[https://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_4]