CODING_STYLE: Mention kernel style in introduction

CODING_STYLE: Mention kernel style in introduction

Currently the coding style guide does not mention that we use kernel
coding style as a base style for LXC.  We have just linked to
CODING_STLYE.md from CONTRIBUTING (which mentions use of kernel coding
style).  We can increase documentation congruence and completeness by
mentioning kernel coding style guide in the introduction to our style
guide.

Add heading and introduction to coding style guide informing readers
that we follow kernel coding style as a base before explaining our style
additions.

Signed-off-by: Tobin C. Harding <me@tobin.cc>