Update README.md

README.md

 - File API supports normal files, Asset files, and CameraRoll files
 - Native-to-native file manipulation API, reduce JS bridging performance loss
 - File stream support for dealing with large file
-- Blob, File, XMLHttpRequest polyfills that make browser-based library available in RN
+- Blob, File, XMLHttpRequest polyfills that make browser-based library available in RN (experimental)
 
 ## TOC
 * [About](#user-content-about)