hawk: Update README with all the new build features
hawk: Update README with all the new build features

We can now specify:

 - Custom kernel headers
 - Custom libbpf path
 - Custom vmlinux

NB in reality these are probably all just directories within a kernel tree. We
should probably just have a variable called KERNEL_TREE that sets all of these
variables up for you.

googleinterns/hawk,googleinterns/hawk