Add details on using a principal for the admin

Add details on using a principal for the admin

When Ipsilon is being installed with IPA, one is most likely going
to use Kerberos to login to Ipsilon as the administrator.  We should
call this out, as the default of 'admin' for the Ipsilon admin user
will conflict with the IPA 'admin' user.  You will be unable to
create a local 'admin' user at this point, requiring you to modify
the sqlite database directly to change the admin user to a full
principal.

I also corrected a typo and wrapped a line that was > 79 chars.

Signed-off-by: Nathan Kinder <nkinder@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>