software/nginx-push-stream add a README introducing this software

software/nginx-push-stream/README.md

+# Nginx Push Stream
+
+This software uses the [HTTP Push Stream](https://www.nginx.com/resources/wiki/modules/push_stream/)
+module of nginx to make a [Server Sent Event](https://html.spec.whatwg.org/multipage/server-sent-events.html)
+server.
+
+Two endpoints are available, published as the following connection parameters:
+
+ - `publisher-url`, which uses`/pub{?id}` format, with `id` being the ID of the channel.
+  Clients can subscribe to this URL and be notified of new messages.
+ - `subscriber-url`, which uses `/sub{/id}` format, with `id` being the ID of the channel.
+  `POST` requests to this URL send new messages to this channel.
+
+