Add shared folder to docker container

[agzimmerman]

The FEniCS docker container makes it really easy to use a shared folder between the host and container. How did they do this? Start here: http://fenics-containers.readthedocs.io/en/latest/introduction.html#running-fenics-in-docker

[agzimmerman]

It turns out that nothing needs to be done with the Dockerfile. This already works. I updated the README to show how to mount a shared folder to the Docker container. Unfortunately if you don't specify a valid path from the container's perspective, docker will just go ahead and enter the container without a shared folder.

[agzimmerman]

For the record, from the current README: " Or run the container with access to a shared folder (shared between the host and the container)

docker run -ti -v $(pwd):/home/dealii/shared zimmerman/peclet:latest

"