add RHEL6 kernel version, and a 3.8 hint to the binaries doc

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #4966 from SvenDowideit/add-kernel-version-info

add RHEL6 kernel version, and a 3.8 hint to the binaries doc