Fixes #14635: add comment to update /etc/ImageMagick-6/policy.xml

Documentation/Installation.rst

@@ -210,6 +210,14 @@ The Typo3 graphic eco-system is not used at all by QFQ.
 
 Usage: :ref:`column-thumbnail`.
 
+.. important::
+
+If there are no thumbnails and you see ``convert-im6.q16: no images defined`` in syslog: Check file
+``/etc/ImageMagick-6/policy.xml`` and update:
+
+  old: <policy domain="coder" rights="none" pattern="PDF" />
+  New: <policy domain="coder" rights="read | write" pattern="PDF" />
+
 Setup
 -----