libbpf: Document API and ABI conventions

Document API and ABI for libbpf: naming convention, symbol visibility,
ABI versioning.

This is just a starting point. Documentation can be significantly
extended in the future to cover more topics.

ABI versioning section touches only a few basic points with a link to
more comprehensive documentation from Ulrich Drepper. This section can
be extended in the future when there is better understanding what works
well and what not so well in libbpf development process and production
usage.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>