Updated README.md

README.md

 # About
-This repository contains a Dockerfile for building a base container image for a
-variety of uses in the context of EIC studies at ANL.
+This container image is a base container image for a variety of uses in the
+context of EIC studies at ANL.
 
 ## Key software packages
 * ProIO
 * ROOT
 
-# Using the image
+# Running a container
 ## Docker
 Example:
 ```shell
@@ -36,6 +36,12 @@ seems that the environment is being negatively affected by the user's
 singularity exec base.img bash --norc
 ```
 
+# Using the container
+The main functionality that this container image provides is the ability to
+read and write files that are used in the simulation and reconstruction cycle.
+For this purpose, the image includes ProIO (all languages) and ROOT.  Try
+commands like `proio-ls` or `root`.
+
 # Building the image
 Example:
 ```shell