Docs: The Alda Command-Line Client

[daveyarwood]

See #8 for context.

This should be done by modifying the docs folder in alda-lang/alda. The docs are kept in sync with releases and used to generate http://alda.readthedocs.io, which will be CNAMEd to docs.alda.io.

Note: no docs currently exist for the Alda CLI client -- this is new territory. I think it would be good to include some practical examples of the Alda command-line workflow using the alda client, some information about Alda server and worker processes, how to play files or strings of code, how to start the REPL, etc.