bpf, doc: add faq about bpf development process

In the same spirit of netdev FAQ, start a BPF FAQ as a collection
of expectations and/or workflow details in the context of BPF patch
processing.
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>