diff --git a/README.md b/README.md
index 0bbac7a55c..5df8b4332b 100644
--- a/README.md
+++ b/README.md
@@ -107,6 +107,11 @@ You can configure the app by copying `config.sample.json` to
    valid location on this network. This is used as a hint to the user to indicate
    when a valid location has been entered so it's not necessary for this to be
    exactly correct. Optional.
+1. `cross_origin_renderer_url`: URL to a static HTML page hosting code to help display
+   encrypted file attachments. This MUST be hosted on a completely separate domain to
+   anything else since it is used to isolate the privileges of file attachments to this
+   domain. Default: `usercontent.riot.im`. This needs to contain v1.html from
+   https://github.com/matrix-org/usercontent/blob/master/v1.html
 
 Running as a Desktop app
 ========================