From the documentation
NSLocationWhenInUseUsageDescription (String – iOS) describes the
reason why the app accesses the user’s location normally while running
in the foreground. Include this key when your app uses location
services to track the user’s current location directly. This key does
not support using location services to monitor regions or monitor the
user’s location using the significant location change service. The
system includes the value of this key in the alert panel displayed to
the user when requesting permission to use location services.This key is required when you use the requestWhenInUseAuthorization
method of the CLLocationManager class to request authorization for
location services. If the key is not present when you call the
requestWhenInUseAuthorization method without including this key, the
system ignores your request.This key is supported in iOS 8.0 and later. If your Info.plist file
includes both this key and the NSLocationUsageDescription key, the
system uses this key and ignores the NSLocationUsageDescription key.
Read about it here.
I find that the easiest way to add this key to your info.plist is to right click you info.plist and choose
Open As->Source Code
and then add the following in the end before </dict></plist>
<key>NSLocationWhenInUseUsageDescription</key>
<string></string>
If you want you can add a text in between <string></string> that describes to the user why you want to use his/hers location. This text will show up under the default text in the alert.