Add ./HACKING and refer to it from README.md.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Try to create a HACKING link in README.md.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Try again to create a HACKING link in README.md.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>