docs: Document getAsFileSystemHandle()

Author: tomayac

README.md

@@ -96,6 +96,7 @@ Most browser support basic File selection with drag 'n' drop or file input:
 * [File API](https://developer.mozilla.org/en-US/docs/Web/API/File#Browser_compatibility)
 * [Drag Event](https://developer.mozilla.org/en-US/docs/Web/API/DragEvent#Browser_compatibility)
 * [DataTransfer](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer#Browser_compatibility)
+* [DataTransferItem.getAsFileSystemHandle()](https://developer.mozilla.org/en-US/docs/Web/API/DataTransferItem/getAsFileSystemHandle)
 * [`<input type="file">`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file#Browser_compatibility)
 
 For folder drop we use the [FileSystem API](https://developer.mozilla.org/en-US/docs/Web/API/FileSystem) which has very limited support: