Update Moving from J to Namespace.md

Moving from J to Namespace.md

@@ -0,0 +1,32 @@
+## 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.
+
+This would mean you wrap your code in a try catch block and return the error. Someting like this:
+```
+try {
+    // Your code here;
+} catch (\RuntimeException $e) {
+    throw new \Exception($e->getMessage(), 500, $e);
+}
+```
+Or you would manually report an error back like this:
+```
+throw new \Exception(Text::_('Your error message here'), 500);
+```
+
+## More info on backwards compatible issues:
+[https://docs.joomla.org/Potential_backward_compatibility_issues_in_Joomla_4]
+
+## Deprecated JFactory::getDBO(); alternative
+[https://github.com/joomla/joomla-cms/discussions/38111#discussioncomment-3971334]