* Added noteset on Singularity persistant overlay.

singularity/20210629.Singularity-overlay.md

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+Overlay for Singularity Containers
+==================================
+
+"Persistent Overlays"
+https://sylabs.io/guides/3.5/user-guide/persistent_overlays.html
+
+>   "Persistent overlay directories allow you to overlay a writable file
+    system on an immutable read-only container for the illusion of
+    read-write access. You can run a container and make changes, and
+    these changes are kept separately from the base container image."
+
+