(docs) add file progress to messages prop

Author: antoine92190

README.md

@@ -541,7 +541,8 @@ messages="[
         audio: true,
         duration: 14.4,
         url: 'https://firebasestorage.googleapis.com/...',
-        preview: 'data:image/png;base64,iVBORw0KGgoAA...'
+        preview: 'data:image/png;base64,iVBORw0KGgoAA...',
+        progress: 88
       }
     ],
     reactions: {