Update README file to include hasBackButton Android property

Author: Aitor Cubeles Torres

README.md

@@ -148,6 +148,7 @@ Property       | Description
 Property       | Description
 -------------- | ------
 `showTitle` (Boolean)   | Sets whether the title should be shown in the custom tab. [`true`/`false`]
+`hasBackButton` (Boolean)   | Sets a back arrow instead of the default X icon to close the custom tab. [`true`/`false`]
 `toolbarColor` (String)           | Sets the toolbar color. [`gray`/`#808080`]
 `secondaryToolbarColor` (String)  | Sets the color of the secondary toolbar. [`white`/`#FFFFFF`]
 `enableUrlBarHiding` (Boolean)    | Enables the url bar to hide as the user scrolls down on the page. [`true`/`false`]