README: Add '--rm' to docker run invocations

* Remove the container when the `docker` instance exits.  Anyone running
  these commands copy-pasted from the `README` likely doesn't care about
  the container persisting.
* Keeps the `docker ps -a` output cleaner. :)
exiting early if container is not found
Merge pull request #10 from bobrik/patch-1

Exiting early if container is not found
Merge pull request #9 from kylemanna/docs

README: Add '--rm' to docker run invocations