* Added note on storage overlay on Singularity.

containers/20210629.Singularity-overlay.txt

@@ -0,0 +1,12 @@
+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."
+
+