Initial hacker documentation

This is putative hacker documentation and I'm happy to adjust it as necessary to suit the current maintainers.

This was such a good resource in Docker that it would be a shame to lose it. Unfortunately, we can't just link to the corresponding information in the Docker project as a number of aspects are bound to be different here.

I've assumed that libcontainer will have @crosbymichael as a BDFL, so if the maintainers aren't comfortable with that, we'll have to find another decision-making process.

There are also some placeholder (i.e. broken) links. Hope that's ok for now.

Docker-DCO-1.1-Signed-off-by: Glyn Normington <gnormington@gopivotal.com> (github: glyn)
I've cut it down based on feedback from @rjnagal. I'm happy to delete the whole roadmap if there's a consensus, but I was nervous about removing anything from the libcontainer roadmap that was essential to docker's current roadmap.