document attributes

README.md

@@ -16,6 +16,18 @@ The add-on is still pending review as the time of writing, you can go to
 `about:debugging`, click "This Firefox" and use "Load Temporary Add-on" to load
 `addon/manifest.json` to install the add-on in the meantime.
 
+## Attributes
+
+Originfox will set the following attributes on newly download files:
+
+- `user.xdg.origin.url`: the source url of the download.
+- `user.xdg.referrer.url`: the url the download was initiated from.
+
+For example, if you browsing `https://example.com/images` and from there
+download the image `https://example.com/img/example.png`. Then
+`user.xdg.origin.url` will be set to `https://example.com/img/example.png` and
+`user.xdg.referrer.url` to `https://example.com/images`.
+
 ## Privacy concerns
 
 Note that the the information this stores with the files is potentially