Update auth_guide.md

adding first draft of gswauth section to authentication guide

Change-Id: I801c9f4add18a5e5f5c735e61cf99fc3a5b935c2
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/6222
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/6259