Fix min kernel vsn in README.md

Apparently, CO-RE is only >= 5.5. For now, let's bump the kernel vsn.
Later on, we could try to bring back bpf_probe_read()+__sk_buff. The
latter is a stable version of sk_buff created by the verifier.

Also, update the default usage output.

Reported-by: Vincent Li <vincent.mc.li@gmail.com>
Signed-off-by: Martynas Pumputis <m@lambda.lt>

Cleanup README.md

There are more CAPs which are required to run pwru in the container.
Let's keep "--privileged" for now until we have sorted out CAPs.

Signed-off-by: Martynas Pumputis <m@lambda.lt>