Adding Dockerfile and update Readme.md to support Dockerbuilds

To support Opentxs in Docker, Added a Dockerfile to the root of the repo. 
Where does this help, with individal users wishing to get started with OT quickly, for future deployment and CI systems to build and run a opentxs in a docker container. 
The image is built using docker and copies the local source into a running docker container. 
Here is how to get started;

    e.g, git clone https://github.com/grealish/opentxs.git
    cd opentxs
    docker build .
    docker -i -t run <the build image ID> /bin/bash << any command you want to run or opentxs
    otuser@<continaerID>$

I have uploaded already a build image from my fork if you want to see

    docker run -i -t run grealish/opentxs-lib-cli /bin/bash