Add doc about persistence and docker-compose (#24)

* Update README documentation

Update documentation about persisting data.
Add documentation about using docker-compose

* Tweak wording

README.md

@@ -12,6 +12,12 @@ This currently is pretty minimal and uses:
 * cron
 * vim editor
 
+## Persisting data
+
+To save the Grav site data to the host file system (so that it persists even after the container has been removed), simply map the container's `/var/www/html` directory to a named Docker volume or to a directory on the host.
+
+> If the mapped directory or named volume is empty, it will be automatically populated with a fresh install of Grav the first time that the container starts. However, once the directory/volume has been populated, the data will persist and will not be overwritten the next time the container starts.
+
 ## Building the image from Dockerfile
 
 ```
@@ -31,3 +37,25 @@ Point browser to `http://localhost:8000` and create user account...
 ```
 docker run -d -p 8000:80 --restart always -v grav_data:/var/www/html grav:latest
 ```
+
+## Running Grav Image with docker-compose and a volume mapped to a local directory
+
+Running `docker-compose up -d` with the following docker-compose configuration will automatically build the Grav image (if the Dockerfile is in the same directory as the docker-compose.yml file). Then the Grav container will be started with all of the site data persisted to a named volume (stored in the `./grav` directory.
+
+```.yml
+volumes:
+  grav-data:
+    driver: local
+    driver_opts:
+      type: none
+      device: $PWD/grav
+      o: bind
+
+services:
+  grav:
+    build: ./
+    ports:
+      - 8080:80
+    volumes:
+      - grav-data:/var/www/html
+```