os: add section on visudo usage to adding-users doc (#898)
os: add section on visudo usage to adding-users doc (#898)

* os: add section on visudo usage to adding-users doc

Visudo on CoreOS requires the file be specified or an error is thrown.
Issue: https://github.com/coreos/bugs/issues/1617

* os: edit to adding-users.md with grammar and syntax changes requested by crawford/josh

jonboulle/docs,coreos/docs,endocode/coreos-docs,atxwebs/docs,joshix/docs