Create README for NGINX Proxy (home-assistant#751)

* Create README.md

* ✏️ Tweaks and standarization

nginx_proxy/README.md

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+# Hass.io Core Add-on: NGINX Home Assistant SSL proxy
+
+Sets up an SSL proxy with NGINX and redirects traffic from port 80 to 443.
+
+![Supports aarch64 Architecture][aarch64-shield] ![Supports amd64 Architecture][amd64-shield] ![Supports armhf Architecture][armhf-shield] ![Supports armv7 Architecture][armv7-shield] ![Supports i386 Architecture][i386-shield]
+
+## About
+
+Sets up an SSL proxy with NGINX web server. It is typically used to forward SSL internet traffic while allowing unencrypted local traffic to/from a Home Assistant instance.
+
+Make sure you have generated a certificate before you start this add-on. The [Duck DNS](https://github.com/home-assistant/hassio-addons/tree/master/duckdns) add-on can generate a Let's Encrypt certificate that can be used by this add-on.
+
+## Installation
+
+The installation of this add-on is straightforward and easy to do.
+
+1. Navigate in your Home Assistant frontend to **Hass.io** -> **Add-on Store**.
+2. Find the "NGINX Home Assistant SSL proxy" add-on and click it.
+3. Click on the "INSTALL" button.
+
+## How to use
+
+The NGINX Proxy add-on is commonly used in conjunction with the [Duck DNS](https://github.com/home-assistant/hassio-addons/tree/master/duckdns) add-on to set up secure remote access to your Home Assistant instance. The following instructions covers this scenario.
+
+1. The certificate to your registered domain should already be created via the [Duck DNS](https://github.com/home-assistant/hassio-addons/tree/master/duckdns) add-on or another method. Make sure that the certificate files exist in the `/ssl` directory.
+2. In the `configuration.yaml` file, some options in the `http:` section are no longer necessary for this scenario, and should be commented out or removed:
+    - `ssl_certificate`
+    - `ssl_key`
+    - `server_port`
+3. Change the `domain` option to the domain name you registered (from DuckDNS or any other domain you control).
+4. Leave all other options as-is.
+5. Save configuration.
+6. Start the add-on.
+7. Have some patience and wait a couple of minutes.
+8. Check the add-on log output to see the result.
+
+## Configuration
+
+Add-on configuration:
+
+```json
+{
+  "domain": "home.example.com",
+  "certfile": "fullchain.pem",
+  "keyfile": "privkey.pem",
+  "hsts": "max-age=31536000; includeSubDomains",
+  "customize": {
+    "active": false,
+    "default": "nginx_proxy_default*.conf",
+    "servers": "nginx_proxy/*.conf"
+  }
+}
+```
+
+### Option: `domain` (required)
+
+The domain name to use for the proxy. 
+
+### Option: `certfile` (required)
+
+The certificate file to use in the `/ssl` directory. Keep filename as-is if you used default settings to create the certificate with the [Duck DNS](https://github.com/home-assistant/hassio-addons/tree/master/duckdns) add-on.
+
+### Option: `keyfile` (required)
+
+Private key file to use in the `/ssl` directory. 
+
+### Option: `hsts` (required)
+
+Value for the [`Strict-Transport-Security`][hsts] HTTP header to send. If empty, the header is not sent.
+
+### Option `customize.active` (required)
+
+If true, additional NGINX configuration files for the default server and additional servers are read from files in the `/share` directory specified by the `default` and `servers` variables.
+
+### Option `customize.default` (required)
+
+The filename of the NGINX configuration for the default server, found in the `/share` directory.
+
+### Option `customize.servers` (required)
+
+The filename(s) of the NGINX configuration for the additional servers, found in the `/share` directory.
+
+## Known issues and limitations
+
+- By default, port 80 is disabled in the add-on configuration in case the port is needed for other components or add-ons like `emulated_hue`. 
+
+## Support
+
+Got questions?
+
+You have several options to get them answered:
+
+- The [Home Assistant Discord Chat Server][discord].
+- The Home Assistant [Community Forum][forum].
+- Join the [Reddit subreddit][reddit] in [/r/homeassistant][reddit]
+
+In case you've found a bug, please [open an issue on our GitHub][issue].
+
+[aarch64-shield]: https://img.shields.io/badge/aarch64-yes-green.svg
+[amd64-shield]: https://img.shields.io/badge/amd64-yes-green.svg
+[armhf-shield]: https://img.shields.io/badge/armhf-yes-green.svg
+[armv7-shield]: https://img.shields.io/badge/armv7-yes-green.svg
+[discord]: https://discord.gg/c5DvZ4e
+[forum]: https://community.home-assistant.io
+[hsts]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security
+[i386-shield]: https://img.shields.io/badge/i386-yes-green.svg
+[issue]: https://github.com/home-assistant/hassio-addons/issues
+[reddit]: https://reddit.com/r/homeassistant
+[repository]: https://github.com/hassio-addons/repository