refs #955: add upgrade to v4.0 notes

Author: ipalo

UPGRADE-4.0.md

@@ -3,3 +3,9 @@
 ### ResultPager
 
 * `\Github\ResultPagerInterface::postFetch` is deprecated, and the method will be removed from the ResultPager interface/class.
+
+### Authentication methods
+
+* `Github\Client::AUTH_CLIENT_ID` is deprecated, use `Github\AuthMethod::CLIENT_ID` instead.
+* `Github\Client::AUTH_ACCESS_TOKEN` is deprecated, use `Github\AuthMethod::ACCESS_TOKEN` instead.
+* `Github\Client::AUTH_JWT` is deprecated, use `Github\AuthMethod::JWT` instead.