Created DNS-01 hook for DuckDNS (markdown)

DNS-01-hook-for-DuckDNS.md

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+The following hook will work for the DNS-01 challenge with [duckdns.org](https://www.duckdns.org)
+
+```bash
+#!/usr/bin/env bash
+
+#
+# dns-01 challenge for DuckDNS
+# https://www.duckdns.org/spec.jsp
+
+set -e
+set -u
+set -o pipefail
+
+domain="your-domain"
+token="your-token"
+
+case "$1" in
+    "deploy_challenge")
+        curl "https://www.duckdns.org/update?domains=$domain&token=$token&txt=$4"
+        echo
+        ;;
+    "clean_challenge")
+        curl "https://www.duckdns.org/update?domains=$domain&token=$token&txt=removed&clear=true"
+        echo
+        ;;
+    "deploy_cert")
+        ;;
+    "unchanged_cert")
+        ;;
+    "startup_hook")
+        ;;
+    "exit_hook")
+        ;;
+    *)
+        echo Unknown hook "${1}"
+        exit 0
+        ;;
+esac
+```