Add "Introduction" section of the SHEEPDOG ADMINISTRATOR GUIDE.

Text now is no longer than 80 characters.
Other small chenges have been made.

git send-email --to=sheepdog@lists.wpkg.org

Signed-off-by: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>