Add a spec to the list

doc/src/guide/specs.asciidoc

@@ -89,6 +89,7 @@ for or related to HTTP.
 * https://tools.ietf.org/html/rfc8297[RFC 8297]: An HTTP Status Code for Indicating Hints
 * https://tools.ietf.org/html/rfc8336[RFC 8336]: The ORIGIN HTTP/2 Frame
 * 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://www.w3.org/TR/webmention/[Webmention]: Webmention
 
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