selftests/bpf: Document vmtest.sh dependencies

Add a list of vmtest script dependencies to make it easier for new
contributors to get going.

Signed-off-by: Evgeniy Litvinenko <evgeniyl@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210723223645.907802-1-evgeniyl@fb.com