add a link to the security documentation when we mention the docker group (or -G)

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #4784 from SvenDowideit/reference-security-doc

add a link to the security documentation when we mention the docker group (or -G)