Add RFC 8586

doc/src/guide/specs.asciidoc

@@ -91,6 +91,7 @@ for or related to HTTP.
 * https://tools.ietf.org/html/rfc8441[RFC 8441]: Bootstrapping WebSockets with HTTP/2
 * https://tools.ietf.org/html/rfc8470[RFC 8470]: Using Early Data in HTTP
 * https://tools.ietf.org/html/rfc8473[RFC 8473]: Token Binding over HTTP
+* https://tools.ietf.org/html/rfc8586[RFC 8586]: Loop Detection in Content Delivery Networks (CDNs)
 * https://www.w3.org/TR/webmention/[Webmention]: Webmention
 
 ==== Upcoming