Add README.md to archive directory

archive/README.md

+# Archive
+
+This directory archives all of the older, system-specific, Singularity 
+2.x-era container definition files that were created as part of the
+naked-singularity project during its first few years of existance. It
+also stores a copy of the original naked-singularity.sh helper script.
+Neither the original helper script, nor the definitions files stored 
+within this directory will be updated again. As such, they should not be
+used in any production workloads moving forward, unless absolutely 
+necessary. They are stored here simply to serve as a historical 
+reference for the time being.