Add missing description

mix.exs

@@ -4,6 +4,7 @@ defmodule Ejabberd.Mixfile do
   def project do
     [app: :ejabberd,
      version: "16.01.0",
+     description: description,
      elixir: "~> 1.1",
      elixirc_paths: ["lib"],
      compile_path: ".",
@@ -14,6 +15,12 @@ defmodule Ejabberd.Mixfile do
      deps: deps]
   end
 
+  defp description do
+    """
+    Robust, ubiquitous and massively scalable Jabber / XMPP Instant Messaging platform.
+    """
+  end
+
   def application do
     [mod: {:ejabberd_app, []},
      applications: [:ssl],