Add: include correct URL for feed (OpenTTD#157)

This should fix the errors on the feed validator:
https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.openttd.org%2Ffeed.xml

_config.yml

@@ -4,7 +4,7 @@ description: >- # this means to ignore newlines until "baseurl:"
   An open source simulator based on the classic game Transport Tycoon Deluxe. It attempts to mimic the original game as closely as possible while extending it with new features.
 baseurl: ""
 staticurl: /static
-url: ""
+url: "https://www.openttd.org/"
 timezone: Etc/UTC
 excerpt_separator: <!-- more -->
 future: true