README: update documentation

BUG=None
TEST=None

Change-Id: I7b43381cedfabcd73848106e9db36f7a2171614c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/test_planner/+/2610944
Tested-by: Sean Abraham <seanabraham@chromium.org>
Auto-Submit: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Commit-Queue: Dhanya Ganesh <dhanyaganesh@chromium.org>
diff --git a/README.md b/README.md
index 2b08891..a33af14 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 # test\_planner code has moved
 
 The test planner code has been relocated to the chromiumos/infra/go repo.
-See https://chromium.googlesource.com/chromiumos/infra/go/+/master
+See https://chromium.googlesource.com/chromiumos/infra/go
 
-seanabraham will soon be deleting this repo entirely.
+This repo is effectively dead, but it should be kept in existence since
+older manifests may reference it.