Documentation: Introduce coding style

As the number of external contributions to Jailhouse grows, the need
for some formal coding style to ease review process and integration
arise. This is the first attempt to summarize what's have been discussed
on jailhouse-dev@googlegroups.com mailing list so far.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
[Jan: clarify include block separation]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>