Add docs for scheduled refresh (#11)

- Add user agent for feed requests

src/lib/Feed.php

@@ -26,6 +26,10 @@ class Feed {
     /** @var string The XML namespace for XHTML */
     public const string XHTML_NS = 'http://www.w3.org/1999/xhtml';
 
+    /** @var string The user agent for Feed Reader Central's refresh requests */
+    private const string USER_AGENT =
+        'FeedReaderCentral/' . FRC_VERSION . ' +https://bitbadger.solutions/open-source/feed-reader-central';
+
     /**
      * When parsing XML into a DOMDocument, errors are presented as warnings; this creates an exception for them
      *
@@ -170,6 +174,7 @@ class Feed {
         curl_setopt($docReq, CURLOPT_RETURNTRANSFER, true);
         curl_setopt($docReq, CURLOPT_CONNECTTIMEOUT, 5);
         curl_setopt($docReq, CURLOPT_TIMEOUT,        15);
+        curl_setopt($docReq, CURLOPT_USERAGENT,      self::USER_AGENT);
 
         $result = [
             'content' => curl_exec($docReq),