README: mention the git repo

Change-Id: I7c8bfa41bf29c42fa20478f0ba51f5d74f035876
Reviewed-on: https://go-review.googlesource.com/c/debug/+/625675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: ianlancetaylor

README.md

@@ -11,8 +11,10 @@ Most of them are currently are at an early, *experimental* stage.
 ## Report Issues / Send Patches
 
 This repository uses Gerrit for code changes. To learn how to submit changes to
-this repository, see https://golang.org/doc/contribute.html.
+this repository, see https://go.dev/doc/contribute.
+
+The git repository is https://go.googlesource.com/debug.
 
 The main issue tracker for the debug repository is located at
-https://github.com/golang/go/issues. Prefix your issue with "x/debug:" in the
+https://go.dev/issues. Prefix your issue with "x/debug:" in the
 subject line, so it is easy to find.