Add .asf.yaml file

This is a follow-up from the "[NOTICE] Git web site publishing to be
done via .asf.yaml only as of July 1st" email sent to
users@infra.apache.org.
diff --git a/.asf.yaml b/.asf.yaml
new file mode 100644
index 0000000..6e33a03
--- /dev/null
+++ b/.asf.yaml
@@ -0,0 +1,2 @@
+publish:
+    whoami: asf-site