Expand Docker section in README with clone, output details, and country codes

Author: midwire

README.md

@@ -107,27 +107,38 @@ Both `lat` and `lon`, geocodes, are populated for each zipcode record.
 
 ## Docker
 
-If you prefer not to install Ruby locally, you can use Docker to generate the database.
+If you prefer not to install Ruby locally, you can use Docker to generate the database. You only need [Docker](https://docs.docker.com/get-docker/) installed.
 
-**Build the image:**
+### Build the image
 
 ```bash
+$ git clone https://github.com/midwire/free_zipcode_data.git
+$ cd free_zipcode_data
 $ docker build -t free_zipcode_data .
 ```
 
-**Generate data for a single country:**
+### Generate data
+
+Use the `COUNTRY` environment variable to specify a 2-letter country code. Omit it to process all available countries.
+
+**Single country (e.g., US):**
 
 ```bash
 $ docker run --rm -v $(pwd)/output:/output -e COUNTRY=US free_zipcode_data
 ```
 
-**Generate data for all countries:**
+**All countries:**
 
 ```bash
 $ docker run --rm -v $(pwd)/output:/output free_zipcode_data
 ```
 
-The SQLite database and CSV files will be written to the `./output/` directory on your host.
+The following files will be written to the `./output/` directory on your host:
+
+* `free_zipcode_data.sqlite3` - SQLite database with countries, states, counties, and zipcodes tables
+* `countries.csv`, `states.csv`, `counties.csv`, `zipcodes.csv` - CSV exports of each table
+
+Look up supported country codes at [GeoNames](http://download.geonames.org/export/zip/).
 
 ## Data License