Add mention of mounting cgroupfs properly to PACKAGERS.md

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Promote btrfs
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
check if working dir is a directory and raise corresponding errors when making dir.

Docker-DCO-1.1-Signed-off-by: Liang-Chi Hsieh <viirya@gmail.com> (github: viirya)
Fix fish completion when having alias on awk or grep

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume@charmes.net> (github: creack)
upstart: use exec here so upstart can monitor the process and not just a shell

Docker-DCO-1.1-Signed-off-by: Michael Gorsuch <michael.gorsuch@gmail.com> (github: gorsuch)
Merge pull request #4838 from crosbymichael/btrfs-ftw

Promote btrfs
Merge pull request #4879 from gorsuch/exec-upstart

Use "exec" in the upstart script to let upstart manage the docker process instead of a shell
Merge pull request #4818 from viirya/fix_working_dir_not_dir

fix the problem when setting existing file as working dir
Merge pull request #4865 from creack/fix_fish_completion

Fix fish completion when having alias on awk or grep
Merge pull request #4613 from tianon/add-cgroupfs-mention-to-packagers

Add mention of mounting cgroupfs properly to PACKAGERS.md