Fix(readme): include instructions for development with Docker container via file mounts

[rsutton1]

The README doesn't include instructions for development through the Docker container. Currently, development requires either:

1. running Salt on the host, which can cause reproducibility issues across OS's (e.g. when developing on Windows)
2. iterating over the Packer file which can cause a slow iteration cycle for certain Packer builds (i.e. devbox-onebuild)

Development over the Docker container would significantly improve iteration time.