Merge pull request #172 from adjust/adid_readme

Add device Id's to README

Author: nonelse

README.md

@@ -493,6 +493,16 @@ ADJEvent *event = [ADJEvent eventWithEventToken:@"abc123"];
 You can read more about special partners and these integrations in our
 [guide to special partners.][special-partners]
 
+### 13. Device IDS
+
+Certain services (such as Google Analytics) require you to coordinate Device and Client IDs in order to prevent duplicate reporting. 
+
+To obtain the device identifier IDFA, call the function `idfa`:
+
+```objc
+NSString * idfa = [Adjust idfa];
+```
+
 [adjust.com]: http://adjust.com
 [cocoapods]: http://cocoapods.org
 [carthage]: https://github.com/Carthage/Carthage