cmd/nginx-auth: add experimental status badge to README
# nginx-auth
+[![status: experimental](https://img.shields.io/badge/status-experimental-blue)](https://tailscale.com/kb/1167/release-stages/#experimental)
+
This is a tool that allows users to use Tailscale Whois authentication with
NGINX as a reverse proxy. This allows users that already have a bunch of
services hosted on an internal NGINX server to point those domains to the
