version: delete generator, add README

Updates golang/go#23223

Change-Id: I98eb37fc572235ad4eb6e0bd6e687c308c99b4a9
Reviewed-on: https://go-review.googlesource.com/123678
Reviewed-by: Andrew Bonventre <[email protected]>

Author: bradfitz

version/README.md

@@ -0,0 +1,4 @@
+# Deprecated.
+
+These are kept for compatibility, but new versions now live in
+the "dl" repo: `go get -d golang.org/dl/...` or `git clone https://go.googlesource.com/dl`.