Add some docs in README. Add demo gif.

Dockerfile

@@ -1,3 +1,6 @@
+# To run spidy in a container and write all files back to the host filesystem:
+#   docker run --rm -it -v $PWD:/data spidy
+
 FROM python:3.6
 LABEL maintainer "Peter Benjamin <petermbenjamin@gmail.com>"
 WORKDIR /src/app/

README.md

@@ -102,6 +102,20 @@ Here are some features we figure are worth noting.
 
 # Tutorial
 
+## Using with Docker
+Spidy can be easily run in a Docker container.<br>
+
+- First, build the [`Dockerfile`](dockerfile): `docker build -t spidy .`
+  - Verify that the Docker image has been created: `docker images`
+- Then, run it: `docker run --rm -it -v $PWD:/data spidy`
+  - `--rm` tells Docker to clean up after itself by removing stopped containers.
+  - `-it` tells Docker to run the container interactively and allocate a pseudo-TTY.
+  - `-v $PWD:/data` tells Docker to mount the current working directory as `/data` directory inside the container. This is needed if you want Spidy's files (e.g. `crawler_done.txt`, `crawler_words.txt`, `crawler_todo.txt`) written back to your host filesystem.
+
+### Spidy Docker Demo
+
+![Spidy Docker Demo](media/spidy_docker_demo.gif)
+
 ## Installing from PyPI
 Spidy can be found on the Python Package Index as `spidy-web-crawler`.<br>
 You can install it from your package manager of choice and simple run the `spidy` command.<br>