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