vgo: add doc comment

main.go

diff --git a/main.go b/main.go
index dde0b98..c956185 100644
--- a/main.go
+++ b/main.go
@@ -2,6 +2,27 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Vgo is a prototype of what the go command
+// might look like with integrated support for package versioning.
+//
+// Download and install with:
+//
+//	go get -u golang.org/x/vgo
+//
+// Then run "vgo" instead of "go".
+//
+// See https://research.swtch.com/vgo-intro for an overview
+// and the documents linked at https://research.swtch.com/vgo
+// for additional details.
+//
+// This is still a very early prototype.
+// You are likely to run into bugs.
+// Please file bugs in the main Go issue tracker,
+// https://golang.org/issue,
+// and put the prefix `x/vgo:` in the issue title.
+//
+// Thank you.
+//
 package main
 
 import Main "cmd/go"