php: add README.md explaining the configuration

It was lost in the `Not you: team` change. It probably makes sense to
keep this documented as people involved in this project might change
over time and sometimes memory can also be blurry :)

php/README.md

@@ -0,0 +1,38 @@
+# Webhook Receiver
+
+This PHP code receives the GitHub webhook, checks them for integrity and publishes messages on rabbitmq.
+
+
+## Configuration
+
+The code expects a `config.php` in it's parent directory. An example configuration looks like this:
+
+```php
+<?php
+
+require_once __DIR__ . '/vendor/autoload.php';
+use PhpAmqpLib\Connection\AMQPSSLConnection;
+use PhpAmqpLib\Message\AMQPMessage;
+
+function rabbitmq_conn($timeout = 3) {
+    $host = 'events.nix.gsc.io';
+    $connection = new AMQPSSLConnection(
+        $host, 5671,
+        'eventsuser, eventspassword, '/',
+        array(
+            'verify_peer' => true,
+            'verify_peer_name' => true,
+            'peer_name' => $host,
+            'verify_depth' => 10,
+            'ca_file' => '/etc/ssl/certs/ca-certificates.crt',
+        ), array(
+            'connection_timeout' => $timeout,
+        )
+    );
+    return $connection;
+}
+
+function gh_secret() {
+    return "github webhook secret";
+}
+```