README: explain that repository is obsolete

Remove reference cloning repo into GOPATH.

For golang/go#62645

Change-Id: Id1b483f63e7a1c82529df5d991179c07acd51245
Reviewed-on: https://go-review.googlesource.com/c/vgo/+/624217
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>

README.md

@@ -1,3 +1,8 @@
+# Obsolete
+
+This repository is obsolete. All features have been incorporated into
+the main Go tools. The following is of historical interest only.
+
 # Versioned Go Command (vgo)
 
 This repository holds a standalone implementation of a version-aware `go` command,
@@ -13,9 +18,6 @@ the Go 1.11 `go` command, with two changes:
 
 Use `go get -u golang.org/x/vgo`.
 
-You can also manually
-git clone the repository to `$GOPATH/src/golang.org/x/vgo`.
-
 ## Report Issues / Send Patches
 
 See [CONTRIBUTING.md](CONTRIBUTING.md).