Add documentation on task syntax

[idpaterson]

There are countless ways to specify dates with parsedatetime, but a much smaller number of ways to denote recurrence, starred tasks, and list membership. The README instructs the user to learn from the subtitle prompts, but that doesn't help much with figuring out something like the minimal typing necessary to add a recurring task.

Since the README is already a bit long, this would be best under GitHub pages. I have a lot of plans for using pages on this repository and some groundwork for it in grunt, but the going may be slow.