add empty doc.go in module root to support some tooling

Some tools MAY have issues with empty packages.
It may be more problematic if the package is a module root.

To possibly help with this, we will use this doc.go
to make this module root a real package with files.

Updates #291

doc.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2012-2018 Ugorji Nwoke. All rights reserved.
+// Use of this source code is governed by a MIT license found in the LICENSE file.
+
+/*
+Package go is an empty holder package for go-codec.
+
+This doc.go is here just to support tools that have problems with empty packages.
+
+DO NOT USE THIS PACKAGE DIRECTLY.
+*/
+package go