HOWTO for running Rayvens on Docker

This [blog](https://medium.com/codeflare/rayvens-event-sources-sinks-and-event-driven-programming-with-ray-b229794dc41f) explains how to run Rayvens with `docker run`. These instructions should be included in the GitHub documentation.

```
docker run --rm -i -t -v "$PWD"/aapl.py:/home/ray/aapl.py \
  quay.io/ibm/rayvens \
  python aapl.py "$SLACK_CHANNEL" "$SLACK_WEBHOOK"
```