Add README.md file for sysinit directory

Note, this text is just copied from sysinit.go

See Issue #5873

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: https://github.com/timthelion)
Merge pull request #5879 from timthelion/docs-sysinit-readme

Add README.md file for sysinit directory
Add restorecon/matchpathcon/chcon so we can label content on the fly
Automatically relabel volume mounts if user passes :Z on -v command

When you volume mount into a labeled container, we want to allow the
administrator/user the ability to tell docker to fix the labels on the mount.

:Z will set the labels so that only the processes within the container can
read/write content.  Since the labels change on creation, this can not be done
by the administrator prior to running the container

:z will change the labels so all containers could read/write content.