Add "Process downloaded images" option to documentation

Related: #59

DOCUMENTATION.md

@@ -59,6 +59,8 @@ When you're configuring mappings for an Assets field, or a Bard field, a few add
 * **Download when missing?**
     * By default, when the importer comes across an asset it can't find in Statamic, it will skip it.
     * However, if you wish, the importer can download any missing assets for you into the configured asset container.
+* **Process downloaded images?**
+    * Determine whether downloaded images should be processed using the asset container's [source preset](https://statamic.dev/image-manipulation#process-source-images).
 * **Folder**
     * By default, downloaded assets will use same folder structure as the original URL. If you'd like to download them into a specific folder, you can select one here.
 * **Alt Text**