i-g-t has become a fairly big project with lots of people involved, so
lets document the basics and formalize the current process a bit.

Also use this opportunity to announce Thomas Wood as igt maintainer
once more.

v2: Recommend --subject-prefix="PATCH i-g-t" as suggested by Damien.

v3: Clean out contributing-related information from README.

Cc: Thomas Wood <thomas.wood@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>