encoding/textpb: add package doc

Change-Id: I678785de4a227b7df2807b297a7bd40b5459d499
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/166418
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

encoding/textpb/doc.go

@@ -0,0 +1,8 @@
+// Copyright 2019 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.
+
+// Package textpb marshals and unmarshals protocol buffer messages as the
+// textproto format.
+
+package textpb