[enh] add HSTS header (#419)

Caddyfile

@@ -65,6 +65,11 @@ header {
 
 	# Comment header to allow indexing by search engines
 	X-Robots-Tag "noindex, nofollow, noarchive, nositelinkssearchbox, nosnippet, notranslate, noimageindex"
+    
+	# enable HSTS
+	# WARNING: Once this value is set, the site must continue to support HTTPS until the expiry time is reached.
+
+	# Strict-Transport-Security max-age=15768000;
 
 	# Remove "Server" header
 	-Server