docs: add link for idfv

README.md

@@ -707,7 +707,7 @@ const uniqueId = DeviceInfo.getUniqueID();
 
 **Notes**
 
-> * iOS: This is `IDFV` so it will change if all apps from the current apps vendor have been previously uninstalled.
+> * iOS: This is [`IDFV`](https://developer.apple.com/documentation/uikit/uidevice/1620059-identifierforvendor) so it will change if all apps from the current apps vendor have been previously uninstalled.
 > * android: Prior to Oreo, this id ([ANDROID_ID](https://developer.android.com/reference/android/provider/Settings.Secure.html#ANDROID_ID)) will always be the same once you set up your phone.
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