Update upgrade.md
Update upgrade.md

Explains that it is recommended, but not required, to upgrade one minor version at a time.
andrewsykim/kops,chrislovecnm/kops,justinsb/kops,tsupertramp/kops,aledbf/kops,blakebarnett/kops,kubernetes/kops,mad01/kops,gambol99/kops,kubernetes/kops,tsupertramp/kops,andrewsykim/kops,mad01/kops,mad01/kops,mad01/kops,justinsb/kops,justinsb/kops,chrislovecnm/kops,chrislovecnm/kops,gambol99/kops,blakebarnett/kops,tsupertramp/kops,blakebarnett/kops,kubernetes/kops,aledbf/kops,robinpercy/kops,chrislovecnm/kops,tsupertramp/kops,robinpercy/kops,andrewsykim/kops,aledbf/kops,aledbf/kops,blakebarnett/kops,andrewsykim/kops,robinpercy/kops,robinpercy/kops,gambol99/kops,gambol99/kops