Add README.md file for sysinit directory

Note, this text is just copied from sysinit.go

See Issue moby#5873

Docker-DCO-1.1-Signed-off-by: Timothy <[email protected]> (github: https://github.com/timthelion)

Author: timthelion

Diff for: sysinit/README.md

@@ -0,0 +1,4 @@
+Sys Init code
+
+This code is run INSIDE the container and is responsible for setting
+up the environment before running the actual process