docs: improve attr section

Author: Kikobeats

README.md

@@ -107,7 +107,7 @@ You can specify the selector using:
 
 ##### attr
 
-Type: `string`<br>
+Type: `string|array`<br>
 default: `html`
 
 It defines which property from the matched selector should be picked.
@@ -116,6 +116,8 @@ For example, if you want to extract an `img`, probably you are interested in `sr
 
 Not defining the `attr` means you are going to get the `html` of the matched selector.
 
+If you provide a collection, they will be used as fallbacks for getting the value.
+
 ## Cache Support
 
 You can enable cache for saving API calls if they have been previously done