Require signature on protected branch (#18)

diff --git a/.asf.yaml b/.asf.yaml
index 1f7913f..b8577d7 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -12,6 +12,7 @@
     main:
       strict: true
       required_linear_history: false
+      required_signatures: true
   dependabot_alerts:  true
   dependabot_updates: false