add build badge (dart-lang/lints#6)

diff --git a/pkgs/lints/README.md b/pkgs/lints/README.md
index 067fdcf..98fa9f0 100644
--- a/pkgs/lints/README.md
+++ b/pkgs/lints/README.md
@@ -1 +1,3 @@
 This repo contains official Dart and Flutter lint rules.
+
+[![Build Status](https://github.com/dart-lang/lints/workflows/validate/badge.svg)](https://github.com/dart-lang/lints/actions)