Added a message on the application url field to better explain what it does

src/Ombi/ClientApp/src/app/settings/customization/customization.component.html

@@ -12,6 +12,10 @@
             </mat-form-field>
         </div>
         <div class="md-form-field">
+            <mat-hint>The application url should be your Externally Accessible URL for example, your internal URL is http://192.168.1.50/ but your Externally
+                Accessible URL is 'https://mydomain.com/requests' Please ensure this field is correct as it drives a lot of functionality include the QR code for the 
+                mobile app and it affects the way email notifications are sent.
+            </mat-hint>
             <mat-form-field appearance="outline">
                 <mat-label>Application URL</mat-label>
                 <input matInput [(ngModel)]="settings.applicationUrl">