Update 6.0-to-6.5.md

A lot of users get authentication errors after this upgrade. Reason being the change in 6.5 that secure cookies when using https. A link to this would be very handy for other upgraders!

https://github.com/gitlabhq/gitlab-public-wiki/wiki/Trouble-Shooting-Guide#cant-verify-csrf-token-authenticitycant-get-past-login-pageredirected-to-login-page

doc/update/6.0-to-6.5.md

@@ -125,3 +125,6 @@ Follow the [`upgrade guide from 5.4 to 6.0`](5.4-to-6.0.md), except for the data
 cd /home/git/gitlab
 sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
 ```
+
+## Login issues after upgrade?
+If running in https mode, be sure to read [Can't Verify csrf token authenticity](https://github.com/gitlabhq/gitlab-public-wiki/wiki/Trouble-Shooting-Guide#cant-verify-csrf-token-authenticitycant-get-past-login-pageredirected-to-login-page)