Add missing global variable definition SATURNIN_SUBCOMMAND

Mostly for purpose of documentation.

src/saturnin.sh.skel

 #
 SATURNIN_META_HELP="${SATURNIN_META_HELP:-}"
 
+#
+# Current running subcommand
+#
+# The value is set and exported into sub-command binary at moment of
+# saturnin__main() call.Z
+#
+SATURNIN_SUBCOMMAND="${SATURNIN_SUBCOMMAND:-}"
+
 saturnin__bug() {
     #
     # Warn about bug in your software