Document how to upgrade node

[grtjn]

If you have older node pre-installed, yum install nodejs sometimes sticks to older version. You need additional steps:

/bin/rm -rf /var/cache/yum /usr/bin/yum remove -y nodejs /bin/rm /etc/yum.repos.d/nodesource* /usr/bin/yum clean all /usr/bin/curl --silent --location https://rpm.nodesource.com/setup_8.x | bash -