Update CHANGELOG for facebookarchive#815 change

Author: SammyK

CHANGELOG.md

@@ -8,6 +8,7 @@ Version 5 of the Facebook PHP SDK is a complete refactor of version 4. It comes
 
 - 5.7.0 (2018-00-00)
   - Add `joined` to list of fields to be cast to `\DateTime` (#950)
+  - Add `GraphPage::getFanCount()` to get the number of people who like the page (#815)
 - 5.6.3 (2018-07-01)
   - Add fix for countable error in PHP 7.2 (originally #969 by @andreybolonin)
 - 5.6.2 (2018-02-15)