Document nghttpx server push feature

doc/nghttpx.h2r

@@ -42,6 +42,35 @@ SIGUSR2
   After new process comes up, sending SIGQUIT to the original process
   to perform hot swapping.
 
+SERVER PUSH
+-----------
+
+nghttpx supports HTTP/2 server push in default mode.  nghttpx looks
+for Link header field (`RFC 5988
+<http://tools.ietf.org/html/rfc5988>`_) in response headers for
+backend server and extracts URI-reference with parameter
+``rel=preload`` (see `preload
+<http://w3c.github.io/preload/#interoperability-with-http-link-header>`_)
+and pushes those URIs to the frontend client. Here is a sample Link
+header field to initiate server push:
+
+.. code-block:: http
+
+  Link: </fonts/font.woff>; rel=preload
+  Link: </css/theme.css>; rel=preload
+
+Currently, the following restrictions are applied for server push:
+
+1. URI-reference must not contain authority.  If it exists, it is not
+   pushed.  ``/fonts/font.woff`` and ``css/theme.css`` are eligible to
+   be pushed.  ``https://example.org/fonts/font.woff`` and
+   ``//example.org/css/theme.css`` are not.
+
+2. The associated stream must have method "GET" or "POST".  The
+   associated stream's status code must be 200.
+
+These limitations may be loosened in the future release.
+
 SEE ALSO
 --------