Announce upcoming changes due to Singularity Hub retirement

README.md

 # naked-singularity
 
-A repository of definition for building 
+A repository of definition files for building 
 [Singularity](https://sylabs.io/guides/latest/user-guide) containers
 around the software applications, frameworks, and libraries you need to
 run on high-performance computing systems.
@@ -33,7 +33,17 @@ singularity pull shub://mkandes/naked-singularity:ubuntu-18.04
 
 ## Status
 
-A work in progress. 
+A work in progress. But an important note on the immediate future ...
+   
+The future availablity of Singularity Hub is currently unknown and
+may be slated for retirement as early as April 2021. If and when
+Singularity Hub is retired, the current set of naked-sinularity 
+definition files will be affected due to the recent move in the past
+year to using a layered dependency chain of multiple containers built
+and hosted on Singularity Hub. Alternative container build and hosting
+options are currently under consideration. This upcoming change will 
+likely affect how a given container's dependency chain is rebuilt and 
+supported long-term.
 
 ## Contribute