Reorganize the runtime section.

This breaks the runtime.rst into multiple files to make it easier
to deal with and as a start on improving the overall structure
of the Hacker's Guide.

It also removes some stubbed out sections that hadn't been written.
Update contribution guidelines.

This points people at the more official document that was added
to better integrate with GitHub.

It also moves a couple of additional notes from the Hacker's Guide
to that more official document.
Suggest that people recursively clone the repo.

This makes it so that the submodules get pulled correctly.
Begin cleaning up.