README: mention the git repo

Change-Id: I6838338ebb8d2130f1442fc844ac787eeb9c8c1f
Reviewed-on: https://go-review.googlesource.com/c/playground/+/625755
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.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>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/README.md b/README.md
index d3a159e..507145c 100644
--- a/README.md
+++ b/README.md
@@ -79,4 +79,6 @@
 ## Contributing
 
 To submit changes to this repository, see
-https://golang.org/doc/contribute.html.
+https://go.dev/doc/contribute.
+
+The git repository is https://go.googlesource.com/playground.