add example for deploying vm with caddy

examples/threefold/tfgrid3deployer/vm_gw_caddy/Caddyfile

@@ -0,0 +1,17 @@
+:80 { # Set this path to your site's directory.
+root \* /usr/share/caddy
+
+    # Enable the static file server.
+    file_server
+
+    # Another common task is to set up a reverse proxy:
+    # reverse_proxy localhost:8080
+
+    # Or serve a PHP site through php-fpm:
+    # php_fastcgi localhost:9000
+
+}
+
+# Refer to the Caddy docs for more information:
+
+# https://caddyserver.com/docs/caddyfile