Add "Should-Start" cgroupfs-mount and cgroup-lite to sysvinit-debian init script

It's fine to list both here because "Should-Start" is a loose binding (ie, if the listed service exists, it'll be started first, but otherwise, this one will start without it).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
remove addString and replace Tail

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Minor fixups to the Docs README

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
process directories in parallel in ChangesDirs

This commit lowers the total time spent in ChangesDirs to half during a
commit.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5657 from tianon/sysvinit-should-start

Add "Should-Start" cgroupfs-mount and cgroup-lite to sysvinit-debian init script
Move cgroups package into libcontainer
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5772 from unclejack/improve_commit_performance

process directories in parallel in ChangesDirs
Remove the cgroups maintainer file

We don't need this because it is covered by the libcontainer MAINTAINERS
file
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5809 from crosbymichael/move-cgroups

Move cgroups package into libcontainer
Updated a bunch of formatting in the docs/sources/use files

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Merge pull request #5801 from jamtur01/fixups

Minor fixups to the Docs README
Add some more specific help for Dockerfile build contexts.

I've copy and pasted the error message so its googlable.

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #5800 from jamtur01/paras

Updated a bunch of formatting in the docs/sources/use files
Merge pull request #5706 from vieux/remove_add_string
Merge pull request #5762 from SvenDowideit/pr_out_add_some_more_specific_help_for_dockerfile_build_contexts

Add some more specific help for Dockerfile build contexts.
docs: explain when RUN cache gets invalidated

This adds a few lines to the RUN Dockerfile docs to explain how to
bypass the RUN caching and how ADD can also invalidate the cache for
these instructions.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5816 from unclejack/docs_run_cache

docs: explain when RUN cache gets invalidated