Add a SECURITY.md file with hints for security researchers and other
parties who might report potential security vulnerabilities.

(From OE-Core rev: 4895e1892a49417fc5a806bd02c1bbac01f37253)

Signed-off-by: Marta Rybczynska <marta.rybczynska@syslinbit.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>