Rename to upgrade guide

Author: marc1706

development/development/changes.rst

@@ -1,8 +1,14 @@
-================
-Changes in phpBB
-================
+=============
+Upgrade Guide
+=============
 
-This document outlines changes introduced in different versions of phpBB. These changes may require adjustments to your existing code.
+This document outlines changes introduced in different versions of phpBB. These changes may require adjustments to your
+existing custom PHP code or Extension code when upgrading to specific versions from a prior versions of phpBB.
+It is mainly meant to be used by core and Extension developers and does not provide end user documentation for upgrading.
+
+Unless explicitly stated otherwise, each version describes the changes compared to the previous major or minor version
+that directly preceded it. For example the upgrade guide for 4.0 will describe the changes compared to 3.3 whereas an upgrade guide
+for 4.1 will describe the changes compared to 4.0.
 
 Contents: