I have split the main body of the CONTRIBUTING file to 3 sections:
 * the short welcome message
 * the code - styling, suggestions
 * sending patches - licensing informations, mailing lists, reviews, etc.

Changes for the code section contents:
 * link to the kernel coding style docs
 * be more clear on subtest naming
 * mention igt_describe()

v2: Add link to the online documentation for igt-describe. It will start
    working once the series is merged. (Simon)

Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Simon Ser <simon.ser@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Simon Ser <simon.ser@intel.com>