Info/user guide page

Many of Prat's awesome features are completely undiscoverable, and others may not be completely obvious. Let's make a little guide/reference pane for folks and keep it up to date with current features.

1. Point out how to add/remove/reorder channels
2. Quick guide to our markdown variant
3. `@username` callouts and their behavior (also mention tab-completion).
4. `#foobar` room links
5. Shortcuts (at least point out that `?` will show the shortcut list, or this can be part of the same info pane)
6. Explain what 'away' means