Extend usage message with info on config and logging

bin/notifirc

     #
     local self      # our name
     self=$(basename "$0")
-    echo "usage: $self [options] message" >&2
-    echo "usage: $self [options] [-l max_lines] [-L max_width] -f message_file|-" >&2
-    echo "options: [-h host] [-p port] [-n nick] [-c context] [-u user]" >&2
+    {
+        echo "usage: $self [options] message"
+        echo "usage: $self [options] [-l max_lines] [-L max_width] -f message_file|-"
+        echo "options: [-h host] [-p port] [-n nick] [-c context] [-u user]"
+        echo ""
+        echo "Defaults for options can be defined as POSIX shell assignments under"
+        echo "/etc/notifirc.rc or ~/.notifirc.rc. (E.g. 'nick=jdoe', 'max_width=40')."
+        echo ""
+        echo "All is logged under /var/log/notifirc.log, ~/.notifirc.log, or"
+        echo "/tmp/notifirc.log, whichever $self can write to."
+    } >&2
     exit 2
 }