Add a related RFC

doc/src/guide/specs.asciidoc

@@ -196,3 +196,4 @@ for or related to HTTP.
 * https://tools.ietf.org/html/rfc8075[RFC 8075]: Guidelines for Mapping Implementations: HTTP to CoAP
 * https://tools.ietf.org/html/rfc8132[RFC 8132]: PATCH and FETCH Methods for CoAP
 * https://tools.ietf.org/html/rfc8323[RFC 8323]: CoAP over TCP, TLS, and WebSockets
+* https://tools.ietf.org/html/rfc8516[RFC 8516]: "Too Many Requests" Response Code for CoAP