Add gen-zaddrs.pl and improve docs

contrib/gen-zaddrs.pl

@@ -0,0 +1,17 @@
+#!/usr/bin/env perl
+# Copyright 2019-2022 The Hush developers
+# Released under the GPLv3
+use warnings;
+use strict;
+
+my $hush    = "./src/hush-cli";
+my $znew    = "$hush z_getnewaddress";
+my $count   = 1;
+my $howmany = shift || 50;
+
+while ($count < $howmany) {
+    my $zaddr   = qx{$znew};
+    chomp($zaddr);
+    print qq{$zaddr\n};
+    $count++;
+}