doc: documentation added for upload directory configuration

Author: mrasif

README.md

@@ -82,9 +82,18 @@ def options
     options={
         response_fields: 'isPrivateFile, tags',
         tags: %w[abc def],
-        use_unique_file_name: false
+        use_unique_file_name: false,
+        folder: "your_directory/"
     }
 end
+
+# If you want to set upload dir then you can use following method or you can also use options method.
+# This method shuld return string
+def store_dir
+    "your_directory/"
+end
+
+
 ```
 
 Then you need to modify your model. for example- if your model name is employee then do these changes