Add missing "ps" requirement to PACKAGERS.md

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add "validate" Makefile target

This was supposed to be part of my previous PR, but somehow got missed.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Introducing spīritus lēnis to fix code highlighting issues due to apostrophe.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Fix footer links. They are pointing to the wrong direzione.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Fix broken /dev/fd in mkimage-arch.sh

Exactly the same problem as dotcloud/docker#2356

Docker-DCO-1.1-Signed-off-by: Lance Chen <cyen0312@gmail.com> (github: Lance0312)
Merge pull request #5311 from ostezer/docs-fix-footer-links

Fix footer links. They are pointing to the wrong direzione.
Merge pull request #5309 from ostezer/docs-fix-apostrophe

Introducing spīritus lēnis to fix code highlighting issues due to apostrophe.
Urgent fix for crazy docs error on volumes.

Elaborate on the volumes example to prevent confusion.

Update: Amend the example so it is executable by the reader.
This includes fixing the erring "/var/logs" statement with "/var/log".

Update: Fix sentence structuring to better suit the 80chrs. limit.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5249 from tianon/make-validate

Add "validate" Makefile target
Merge pull request #5312 from ostezer/docs-illusion-confusion

Urgent fix for crazy docs error on volumes.
Merge pull request #5313 from Lance0312/arch-fd-link

Fix borken /dev/fd in mkimage-arch.sh
Merge pull request #5213 from tianon/missing-ps-requirement

Add missing "ps" requirement to PACKAGERS.md