mobile/cmd/gomobile: initial empty main function

Change-Id: Ieac721b486155679f3de2baa13c69d50d63dce3b
Reviewed-on: https://go-review.googlesource.com/2652
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/main.go

@@ -0,0 +1,13 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Gomobile is a tool for building and running mobile apps written in Go.
+
+The tool is under development and not ready for use.
+*/
+package main
+
+func main() {
+}