Require at least 1 approving reviews for PRs (#305)

Copying the setting-up of airflow repo over to airflow-site repo.

Ref: https://github.com/apache/airflow/pull/12020
diff --git a/.asf.yaml b/.asf.yaml
index 0219e82..c46af6d 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -29,3 +29,8 @@
     squash: true
     merge:  false
     rebase: false
+
+  protected_branches:
+    master:
+      required_pull_request_reviews:
+        required_approving_review_count: 1