Update debian.md

Add some instructions for Debian 7 (aka Wheezy/Stable)

Signed-off-by: Vishal Doshi <vishal.doshi@gmail.com>
Try to go for 80-char lines and a little md wrangling.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #7819 from SvenDowideit/debian-wheezy-notes

Debian wheezy notes - minor md tweaks