Add documentation for WAGTAIL_REDIRECTS_FILE_STORAGE setting (#6114)

docs/reference/settings.rst

@@ -534,3 +534,16 @@ Page locking
 
 ``WAGTAILADMIN_GLOBAL_PAGE_EDIT_LOCK`` can be set to ``True`` to prevent users
 from editing pages that they have locked.
+
+Redirects
+=========
+
+.. code-block:: python
+
+   WAGTAIL_REDIRECTS_FILE_STORAGE = 'tmp_file'
+
+By default the the redirect importer keeps track of the uploaded file as a temp file, but on certain environments (load balanced/cloud environments), you cannot keep a shared file between environments. For those cases you can use the build in cache to store the file instead.
+
+.. code-block:: python
+
+   WAGTAIL_REDIRECTS_FILE_STORAGE = 'cache'