ios method 'excludeFromBackupKey' is missing from documentation and types

[JNaftali]

The method still exists (https://github.com/joltup/react-native-fetch-blob/blob/master/ios.js#L41), it's just missing from the wiki documentation (https://github.com/joltup/react-native-fetch-blob/wiki/iOS-API) and the typescript definitions (https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-native-fetch-blob/index.d.ts).

Since there's already a jsdoc comment I'm assuming adding docs wouldn't be too arduous, but I'm not sure I'm the best person to fix it - I'm running across it being used in some old code but don't quite understand why it would be used.

[Traviskn]

I don't think there'd be any harm in documenting it, we'd love to get a PR! I also don't know the use case very well, but the comments seem to indicate you can use it to keep iCloud from backing up the file? I imagine you would only need it if you were saving files to a place where iCloud might pick it up, probably a method that is very dependent on the particular use case.