bpf: create samples/bpf/tcp_bpf.readme

Readme file explaining how to create a cgroupv2 and attach one
of the tcp_*_kern.o socket_ops BPF program.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked_by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>