doc: tell rake rubocop is needed
doc: tell rake rubocop is needed

"rake rubocop" is missing comparing of what ran on travis-ci.

Adds it, so contributor will not missing syntax issue with old puppet
version.

voxpupuli/puppet-collectd,voxpupuli/puppet-collectd,archii/puppet-collectd,voxpupuli/puppet-collectd,archii/puppet-collectd,archii/puppet-collectd