tc: bpf: add initial man page

tc: bpf: add initial man page

Add a start of a man-page to the misc section as a reference and
guide on (e)BPF classifier and actions. Given that tc is only tersely
documented, this is provided in the hope that users will have an
easier getting started with tc and (e)BPF. And, that there's now more
incentive for others to also start documenting their classifier and
actions as well. ;)

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>