Add development environment instructions

Added instructions on how to create a development environment to the Contributors Guide.

Docker-DCO-1.1-Signed-off-by: Dave Tucker dave@dtucker.co.uk (github:
dave-tucker)
My preference is to not include sudo here.  Libcontainer is low level enough that it has to run as root and developers should be aware of this.  I believe this is already in the documentation.