Add module docstring

src/shellfu/saturnin_demo_greet.sh

@@ -2,6 +2,13 @@
 
 shellfu import pretty
 
+#
+# Greet something
+#
+# This is an example Shellfu/Bash module.  The purpose is to demonstrate
+# how a simple Shellfu/Bash module could look like.
+#
+
 saturnin_demo_greet__world() {
     #
     # Greet the world