Use an override file for config + update README.

Closes #6

.gitignore

+/docker-compose.override.yml

README.md

 More details will be added here in due course. For now, the full process
 is described in [this blog post](https://www.chameth.com/2016/05/21/docker-automatic-nginx-proxy).
 
-## Adding extra config to Nginx
+## Getting started
+
+The out-of-the-box setup uses [Lexicon](https://github.com/AnalogJ/lexicon)
+to perform DNS updates. This will work if you use one DNS provider for all
+the domains you wish to use, and Lexicon supports that provider. If that
+is the case, then getting started is very easy:
+
+ 1. Copy docker-compose.override.example.yml to docker-compose.override.yml
+ 2. Change the e-mail address, provider, and provider auth details
+ 3. Run `docker-compose up -d`
+
+If you have existing containers with the appropriate labels, the certificates
+will be requested for them straight away, and proxy rules added. To launch
+a new container and have it be proxied, add the following labels:
+
+    com.chameth.proxy=<port>
+    com.chameth.proxy.protocol=<protocol> # defaults to http
+    com.chameth.vhost=<primary vhost>,<secondary vhost>,<...>
+
+For example:
+
+    docker run \
+        --label com.chameth.proxy=80 \
+        --label com.chameth.vhost=example.domain.com \
+        tutum/hello-world
+
+It may take a minute or two for the certificate to be obtained and for
+Nginx to be reconfigured. You can see output from the various tools
+by running `docker-compose logs -f`.
+
+## Advanced / Tips and Tricks
+
+### Adding extra config to Nginx
 
 Out of the box, the Nginx server will only handle HTTPS requests,
 with a very minimal config. The [extra](extra/) directory contains
 Once you have the services running, you can copy additional config
 using the cp command:
 
-```
-docker cp file.conf autoproxy_nginx:/etc/nginx/conf.d/
-```
+    docker cp file.conf autoproxy_nginx:/etc/nginx/conf.d/
 
 The following config files are available in the extra directory:
 
    disable old protocols and ciphers, enable stapling, etc. This will prevent
    access from older browsers and operating systems!
 
-## Hosting static content
+### Hosting static content
 
 If you're serving static content, it's not desirable to have lots of
 instances of nginx running just to handle requests from the proxy.

docker-compose.override.example.yml

+---
+version: '2'
+
+services:
+
+  letsencrypt-lexicon:
+    environment:
+      # For testing purposes, use the Let's Encrypt staging server.
+      # Remove this for production use!
+      - STAGING=yes
+      # The e-mail address to provide to Let's Encrypt.
+      - EMAIL=your@email.addr
+      # The Lexicon provider to use
+      - PROVIDER=cloudflare
+      # Provider-specific authentication details
+      - LEXICON_CLOUDFLARE_USERNAME=your@email.addr
+      - LEXICON_CLOUDFLARE_TOKEN=1234567890123456789012345678901234567890
+

docker-compose.yml

 #   com.chameth.proxy.protocol=http [optional, defaults to http]
 #
 # To prove ownership of domains to Let's Encrypt, we add a DNS entry when
-# required. You will need to configure one of the letsencrypt-* services
-# below to make these changes.
+# required. This requires you to provide authentication details (e-mail
+# address, API key, password, etc). These should be specified in a
+# docker-compose.override.yml file.
 
 version: '2'
 
       - etcd
 
   # letsencrypt-lexicon obtains Let's Encrypt certificates by modifying
-  # DNS records. It supports several major cloud DNS providers. You
-  # need to set the provider and auth tokens below.
+  # DNS records. It supports several major cloud DNS providers.
   letsencrypt-lexicon:
     image: csmith/letsencrypt-lexicon:latest
     container_name: autoproxy_letsencrypt-lexicon
     restart: always
     volumes:
       - letsencrypt-data:/letsencrypt
-    environment:
-      - STAGING=yes
-      - EMAIL=your@email.addr
-      - PROVIDER=cloudflare
-      - LEXICON_CLOUDFLARE_USERNAME=your@email.addr
-      - LEXICON_CLOUDFLARE_TOKEN=1234567890123456789012345678901234567890
 
   # letsencrypt-generic uses a user-defined hook to update DNS entries.
   # You need to supply your own hook, available at /dns/hook. See the
   #  volumes:
   #    - letsencrypt-data:/letsencrypt
   #    - /my/hook/script:/dns/hook
-  #  environment:
-  #    - STAGING=yes
-  #    - EMAIL=your@email.addr
 
   # service-nginx reads proxy information and vhosts from etcd and
   # creates an nginx vhost config to enable SSL-terminated reverse