Merge branch 'bpf-devel-doc'

Daniel Borkmann says:

====================
Few BPF doc updates

Two changes, i) add BPF trees into maintainers file, and ii) add
a BPF doc around the development process, similarly as we have
with netdev FAQ, but just describing BPF specifics. Thanks!
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>