add to documentation: how to access admin features

[mojodna]

As of the upcoming 0.8.0 release of POSM (via posm/posm-admin-ui@2.2.0), the admin sidebar does not include links to "admin" functionality by default.

To access general-purpose admin functionality (backups at the moment), go directly to http://posm.io/posm/admin.

To access network settings, go directly to http://posm.io/posm/settings.

To include links to admin functionality, edit /opt/posm-www/config.json and remove "showAdmin": false,.

Alternately, set posm_show_admin=true in kickstart/etc/settings prior to installation (only relevant when installing from source).

[danbjoseph]

@mojodna, could you alternatively set "showAdmin": true,?

[danbjoseph]

nvm, needed to read the next line.