Merge pull request #579 from riccardonuzz/patch-1

Update README.md

Author: itinance

README.md

@@ -364,6 +364,8 @@ The following constants are available on the `RNFS` export:
 - `ExternalDirectoryPath` (`String`) The absolute path to the external files, shared directory (android only)
 - `ExternalStorageDirectoryPath` (`String`) The absolute path to the external storage, shared directory (android only)
 
+IMPORTANT: when using `ExternalStorageDirectoryPath` it's necessary to request permissions (on Android) to read and write on the external storage, here an example: [React Native Offical Doc] (https://facebook.github.io/react-native/docs/permissionsandroid)
+
 ### `readDir(dirpath: string): Promise<ReadDirItem[]>`
 
 Reads the contents of `path`. This must be an absolute path. Use the above path constants to form a usable file path.