Add 'bundle exec rake cache:clear' to upgrade instructions from 5.4 to 6.0

Created by: jacargentina

As a consequence on the namespaces change from 5.4 to 6.0, after upgrade i had lots of links on the dashboard not working (pointing to projects without the group part on the url)

I solved it by clearing the cache with

bundle exec rake cache:clear

Maybe this can be added to avoid users having the same issue as me.

@randx ?