Add C type rules, adapted from libvirt HACKING. Also include
a description of special QEMU scalar types.

Move typedef rule from CODING_STYLE rule 3 to HACKING rule 6
where it belongs.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

Add a new file, HACKING, in order to collect recurring
issues with submitted patches.

Start with preprocessor rules, adapted from libvirt HACKING.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>