Add info on level, performance fields to README

[jakory]

Currently, there is information missing in the README about how and when to use the level and performance fields in GameCommand and GameState messages, respectively.

These changes would:

• Add necessary information about the level and performance fields to the README.
• Update formatting in the README to be more readable, including:
  • Wrap lines at 80 characters.
  • Put message fields in bulleted lists.
  • Use markdown subheadings.
• In all files, remove trailing whitespace.

[jakory]

Note that #5 depends on this pull request.