media: Documentation: media: Document how to write camera sensor drivers

While we have had some example drivers, there has been up to date no
formal documentation on how camera sensor drivers should be written; what
are the practices, why, and where they apply.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>