go/vcs: add package comment

According to https://golang.org/doc/effective_go.html#commentary,
"Every package should have a package comment."

Updates golang/go#11490.

Author: dmitshur

go/vcs/vcs.go

@@ -2,6 +2,17 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Package vcs exposes functions for resolving import paths
+// and using version control systems (like Mercurial, Git, or Subversion),
+// which can be used to implement behavior consistent with
+// the standard "go get" command.
+//
+// This package is a copy of internal code in package cmd/go/internal/get,
+// modified to make the identifiers exported. It's provided here
+// for developers who want to write tools with similar semantics.
+// It needs to be manually kept in sync with upstream when changes are
+// done to cmd/go/internal/get, see https://golang.org/issues/11490.
+//
 package vcs // import "golang.org/x/tools/go/vcs"
 
 import (