egen_auc_txt.pl: Add a two-line description of what it is

src/gen_auc_txt.pl

@@ -1,5 +1,8 @@
 #!/usr/bin/perl -w
 
+# this script can generate a CSV file with the specified number of
+# entries, which in turn can be used from osmo-auc.
+
 use strict;
 
 my $COUNT = $ARGV[0];