Add link to ASF YAML README in .asf.yaml

.asf.yaml

diff --git a/.asf.yaml b/.asf.yaml
index ab59476..00a1f4a 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -1,3 +1,4 @@
+# https://github.com/apache/infrastructure-asfyaml/blob/main/README.md
 github:
   rulesets:
     - name: "Default Branch Protection"