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
Merge branch 'master' of https://github.com/gitlabhq/gitlabhq into patch-1
Merge pull request #6553 from tehZeno/patch-1

Update 6.0-to-6.5.md