Add new IOS property to ReadMe

Author: ShaneMckenna23

README.md

@@ -159,6 +159,7 @@ Property       | Description
 `modalEnabled` (Boolean)             | Present the **SafariViewController** modally or as push instead. [`true`/`false`]
 `enableBarCollapsing` (Boolean)      | Determines whether the browser's tool bars will collapse or not. [`true`/`false`]
 `ephemeralWebSession` (Boolean)      | Prevent re-use cookies of previous session (openAuth only) [`true`/`false`]
+`formSheetContentSize` (Object)      | Custom size for `formSheet` modals [`{width: 400, height: 500}`]
 
 ### Android Options
 Property       | Description