Add top-level documentation.

Updated README.md.
- Added build/test instructions.
- Added contribution ideas.
- Added udp demo description and example output.

Added top level documentation to all modules.
- sometimes a bit pointless, but Network.Kontiki.Types
  is now relatively well documented with references
  to the paper, sections and ordering of definitions the same as
  exports.

Enabled test coverage with hpc.
- Updated README.md with appropriate instructions.