Add cowboy_req:inform/2,3

User code can now send as many 1xx responses as necessary.

doc/src/guide/resp.asciidoc

@@ -262,6 +262,29 @@ Req = cowboy_req:reply(200, #{
 // example would be automatic concatenation of CSS or JS
 // files.
 
+=== Informational responses
+
+Cowboy allows you to send informational responses.
+
+Informational responses are responses that have a status
+code between 100 and 199. Any number can be sent before
+the proper response. Sending an informational response
+does not change the behavior of the proper response, and
+clients are expected to ignore any informational response
+they do not understand.
+
+The following snippet sends a 103 informational response
+with some headers that are expected to be in the final
+response.
+
+[source,erlang]
+----
+Req = cowboy_req:inform(103, #{
+    <<"link">> => <<"</style.css>; rel=preload; as=style">>,
+    <<"link">> => <<"</script.js>; rel=preload; as=script">>
+}, Req0).
+----
+
 === Push
 
 The HTTP/2 protocol introduced the ability to push resources