Overhaul "debugging" section of README

The "debugging" section of README.md was misleading - it suggested
one needs to build with "mode=debug" to debug (usually, you don't),
or that disabling preemption was a good idea for debugging.

Change this to drop the preempt=0 suggestion, and make mode=debug an
option, not a recommendation. Also link to our "Debugging OSv" wiki
page, which is much more informative - and includes very important
information still missing in the README (such as a line one needs to
add to ~/.gdbinit, and how to handle multiple vcpus).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>