Setting env in nginx: /etc/main.d/*.env not included

The [Setting environment variables](https://github.com/phusion/passenger-docker#setting-environment-variables-in-nginx) section in the README tells users to create `*.env` files to transmit environment variables to the nginx child processes like this:

```
# Dockerfile
ADD postgres.env /etc/nginx/main.d/postgres.env
```

However, the current nginx configuration only includes files in `/etc/nginx/main.d` that end in `*.conf`, and won't include those ending in `*.env`.

```sh
# rgrep main.d /etc/nginx  
/etc/nginx/nginx.conf:include /etc/nginx/main.d/*.conf;
```

This PR updates the README to tell users to inject their environment file as `*-env.conf` so that they are included by nginx, like this:

```
# Dockerfile
ADD postgres-env.conf /etc/nginx/main.d/postgres-env.conf
```