Update README.rst

README.rst

@@ -113,6 +113,14 @@ If you are using Ubuntu 14.04 LTS (trusty), run the following to install the nee
 spdylay is not packaged in Ubuntu, so you need to build it yourself:
 http://tatsuhiro-t.github.io/spdylay/
 
+To enable mruby support for nghttpx, `mruby
+<https://github.com/mruby/mruby>`_ is required.  We need to build
+mruby with C++ ABI explicitly turned on, and probably need other
+mrgems, mruby is manged by git submodule under third-party/mruby
+directory.  Currently, mruby support for nghttpx is disabled by
+default.  To enable mruby support, use ``--with-mruby`` configure
+option.
+
 Building from git
 -----------------