Add another RFC to the list of specs

doc/src/guide/specs.asciidoc

@@ -80,6 +80,7 @@ for or related to HTTP.
 * https://tools.ietf.org/html/rfc7838[RFC 7838]: HTTP Alternative Services
 * https://tools.ietf.org/html/rfc7932[RFC 7932]: Brotli Compressed Data Format
 * https://tools.ietf.org/html/rfc8053[RFC 8053]: HTTP Authentication Extensions for Interactive Clients
+* https://tools.ietf.org/html/rfc8164[RFC 8164]: Opportunistic Security for HTTP/2
 * https://www.w3.org/TR/webmention/[Webmention]: Webmention
 
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