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diff --git a/README.md b/README.md
index 1b60cf2..5da2fa7 100644
--- a/README.md
+++ b/README.md
@@ -153,6 +153,9 @@ var options = {
 };
 ```
 
+The return value of `transformDocument` is used during HTML generation.
+The original document (and any child elements) can be safely modified.
+
 ### API
 
 #### `mammoth.convertToHtml(input, options)`