Added the following docs:
- README.md as index page;
- CONTRIBUTING.md as the contribution guide;
- COPYING file from kernel source;
- CREDITS.md as the credits-file;
- MAILLIST.md as the mailing list info page;
- os.md as the index page of Alibaba Cloud Linux OS.

And also initialized for github pages, cayman is the theme we have
chosen.
Signed-off-by: NCaspar Zhang <caspar@linux.alibaba.com>