Update README.md with command information (#81)
Update README.md with command information (#81)

It wasn't immediately obvious from the README how to access the
commandline or what commands are supported. This changeset adds a brief
overview of the command system and lists current and future commands
based off the command tracking issue [1] at the time of writing.

[1] https://github.com/xi-frontend/xi-term/issues/70
little-dude/xi-tui