Revert "version: delete GENERATE.go."

This reverts commit a447caebf8.

version/GENERATE.go

@@ -0,0 +1,8 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Placeholder that indicates this directory is a valid go package,
+// but that redo must 'redo all' in this directory before it can
+// be imported.
+package version