Add Outlook note to README

README.md

@@ -70,6 +70,16 @@ composer test
    //   3 => string 'TZID=America/Detroit:20160409T090000' (length=36)
    ```
 
+### Are you using Outlook?
+
+Outlook has a quirk where it requires the User Agent string to be set in your request headers.
+
+Using the `httpUserAgent` argument when creating your ICal object solves this problem.
+
+```php
+$ical = new ICal($url, array('httpUserAgent' => 'A User Agent'));
+```
+
 ---
 
 ## When Parsing an iCal Feed