Added radicale link

doc/upgrade.rst

@@ -208,6 +208,12 @@ Required changes to :file:`settings.py`
 
 - Remove ``STATICFILE_DIRS`` variable
 
+Radicale configuration changes
+-----------------------------
+
+You must enable oauth2 authentication in Radicale to make the calendard
+and contact work. Follow this :ref:`guide <radicale_oauth2>`.
+
 Post update
 -----------
 

modoboa/core/checks.py

@@ -41,6 +41,8 @@ E002 = Error(
         "Oauth2 application for radicale has not been found."
         " Calendar and some other features won't be available."
     ),
+    hint=_(
+        "Visit :  https://modoboa.readthedocs.io/en/latest/manual_installation/radicale.html#radicale-oauth2")
     id="modoboa.core.E002",
 )