examples/newswww.psgi: demonstrate standalone NewsWWW usage

Plack::Builder allows "mounting" on with hostnames as well as
path names to enable virtual hosting.  This example demonstrates
how port 80/443 for "news.example.com" can redirect browser
requests when somebody attempts to use a "nntp://" URL and
the software assumes "http://"

diff --git a/examples/newswww.psgi b/examples/newswww.psgi
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+#!/usr/bin/perl -w
+# Copyright (C) 2019 all contributors <meta@public-inbox.org>
+# License: GPL-3.0+ <https://www.gnu.org/licenses/gpl-3.0.txt>
+#
+# NewsWWW may be used independently of WWW.  This can be useful
+# for mapping HTTP/HTTPS requests to the hostname of an NNTP server
+# to redirect users to the proper HTTP/HTTPS endpoint for a given
+# inbox.  NewsWWW exists because people (or software) can mishandle
+# "nntp://" or "news://" URLs as "http://" (or "https://")
+#
+# Usage:
+#      plackup -I lib -o 127.0.0.1 -R lib -r examples/newswww.psgi
+use strict;
+use warnings;
+use Plack::Builder;
+use PublicInbox::WWW;
+use PublicInbox::NewsWWW;
+
+my $newswww = PublicInbox::NewsWWW->new;
+
+# Optional, (you may drop the "mount '/'" section below)
+my $www = PublicInbox::WWW->new;
+$www->preload;
+
+builder {
+       # HTTP/1.1 requests to "Host: news.example.com" will hit this:
+       mount 'http://news.example.com/' => builder {
+               enable 'Head';
+               sub { $newswww->call($_[0]) };
+       };
+
+       # rest of requests will hit this (optional) part for the
+       # regular PublicInbox::WWW code:
+       # see comments in examples/public-inbox.psgi for more info:
+       mount '/' => builder {
+               eval {
+                       enable 'Deflater',
+                               content_type => [ qw(
+                                       text/html
+                                       text/plain
+                                       application/atom+xml
+                                       )]
+               };
+               eval { enable 'ReverseProxy' };
+               enable 'Head';
+               sub { $www->call($_[0]) }
+       };
+}