Openshift install docs: Recommend "add-scc-to-user" over "edit scc"

What does this MR do?

Minor documentation change to Openshift Installation - Security Context Constraints. Recommend using oadm policy add-scc-to-user to anyone using OpenShift v3.1 and above (the majority of people now).

Are there points in the code the reviewer needs to double check?

I tested command changes locally. To confirm, you can eyeball official docs at: https://docs.openshift.com/enterprise/3.1/admin_guide/manage_scc.html#grant-access-to-the-privileged-scc

Why was this MR needed?

Using oadm policy add-scc-to-user is much less error prone and easier to script than manually editing or patching the SCC's with oc edit scc. add-scc-to-user was introduced in v3.1. Unfortunately much third party documentation and comments still suggest manual editing, so this MR will help promote this best practice.