Document the existence of a C ipc project

Long description:

Add a link to https://github.com/acrisci/i3-ipc which is a new ipc
library in the design phase of development. When it is stable, it will
provide bindings to many high-level scripting languages with
GObject-introspection.

This project aims to replace the unmaintained Python library and offer
an ipc library in new languages such as Lua and JavaScript.

To apply this patch, use:
curl http://cr.i3wm.org/patch/390/raw.patch | git am

@@ -760,8 +760,7 @@ all this on your own). This list names some (if you wrote one, please let me

 know):

 C::

-	i3 includes a headerfile +i3/ipc.h+ which provides you all constants.

-	However, there is no library yet.

+	https://github.com/acrisci/i3-ipc (not yet ready for production use)

 Ruby::

 	http://github.com/badboy/i3-ipc

 Perl::