[DOCS] Add docs to disableMMKVBackup functionality

[nazmeln]

I found there's a disableMMKVBackup key in the codebase to disable backup for MMKV files to iCloud, and there's no documentation about it. Could you please add it or it will be removed in future release?

react-native-mmkv-storage/ios/MMKVStorage.mm

Line 59 in 710d4e7

NSString *disableMMKVBackup = [[NSBundle mainBundle].infoDictionary valueForKey:@"disableMMKVBackup"];