add another example to ch-run

sheets/ch-run

@@ -5,3 +5,9 @@
 # Run the given `echo` command inside a Charliecloud container, using the
 # unpacked image at '/data/foo'.
 ch-run /data/foo -- echo hello
+
+# Make the Charliecloud container writable (-w) and mount the host's
+# '/home/user' directory to '/mnt' inside the container using the unpacked 
+# image at '/data/foo'. Run `/mnt/your_exec` inside the container.
+ch-run -w -b /home/user:/mnt /data/foo -- /mnt/your_exec
+