explain iframe configuration (#288)

Author: nworr

source/publish/customization/iframe.rst

@@ -12,6 +12,16 @@ It's possible to embed Lizmap in another webpage by using a iframe. We suggest y
 
 For instance, use this URL : ``/index.php/view/embed/?repository=my_repo&project=my_project``.
 
+Configuration
+-------------
+
+To prevent security issues, you need to explicitly declare websites allowed to embed your Lizmap iframe.
+Fill the :guilabel:`List of websites allowed to embed iframe` field that can be defined for each the repository in :guilabel:`Maps management`.
+
+.. warning::
+
+   Ensure the ``referrer`` is provided to the iframe (you can use the ``referrerpolicy`` attribute : ``<iframe src="....." referrerpolicy="strict-origin-when-cross-origin" ></iframe>`` )
+
 Example
 -------