[Sheepdog] [ANNOUNCE] add Yuan to Sheepdog commiters

[MORITA Kazutaka]

Hi all,

First of all, sorry for my slower response recently.  I'm on vacation
till Jan 3.

During my vacation, it is not good to leave the posted patches
untouched.  Fortunately, Yuan is willing to help me maintain sheepdog,
so I added him to the Sheepdog commiter list.

Yuan is familiar with the details of Sheepdog internal codes and has
sent the most patches these months, so I think he is the most suitable
for the work.  I also think that Yuan should be the maintainer of
Sheepdog storage drivers including "farm" and storage interface
because the most of those codes are written by him.

Let me know if there are any opinions.

Thanks,

Kazutaka