Update MIGRATING.md

MIGRATING.md

@@ -8,7 +8,7 @@ Each of them involves incompatible API changes. The users of google-drive-ruby m
 
 Here are some changes likely affecting google-drive-ruby users:
 
-If you pass an instance of Google::APIClient to GoogleDrive.login_with_oauth, it will no longer work, because Google::APIClient.new was removed. You need to pass credentials generated by [googleauth](https://github.com/google/google-auth-library-ruby) library instead.
+If you pass an instance of Google::APIClient to GoogleDrive.login_with_oauth, it will no longer work, because Google::APIClient.new was removed. See [Authorization document](https://github.com/gimite/google-drive-ruby/blob/master/doc/authorization.md) for new ways to pass credentials.
 
 The field "title" in search queries was renamed to "name". e.g.,