Add upgrade doc

Author: emodric

UPGRADE-4.0.md

@@ -0,0 +1,21 @@
+# UPGRADE FROM 3.x to 4.0
+
+* Support for eZ Platform 3.x has been dropped (and with it support for PHP < 7.4 and Symfony < 5.4)
+
+* Renamed the prefix for simplified role check from `ez:` to `ibexa:`
+
+  **Before**
+
+  ```
+  {% if is_granted('ez:user:register') %}
+      ...
+  {% endif %}
+  ```
+
+  **After**
+
+  ```
+  {% if is_granted('ibexa:user:register') %}
+      ...
+  {% endif %}
+  ```