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diff --git a/README.md b/README.md
index b0d6d49..79a49b3 100644
--- a/README.md
+++ b/README.md
@@ -230,6 +230,18 @@ var options = {
 };
 ```
 
+#### External file access
+
+Source documents may reference files outside of the source document.
+To disable access to any such external files during the conversion process,
+set `options.externalFileAccess` to `false`:
+
+```javascript
+var options = {
+    externalFileAccess: false
+};
+```
+
 #### Custom image handlers
 
 By default, images are converted to `<img>` elements with the source included inline in the `src` attribute.
@@ -536,6 +548,9 @@ For instance:
   and embed the HTML into your website,
   this may allow arbitrary files on the server to be read and exfiltrated.
 
+  To disable access to any such external files during the conversion process,
+  set `options.externalFileAccess` to `false`.
+
 ### Document transforms
 
 **The API for document transforms should be considered unstable,