Improve documentation on how to use within a route

[lukemelia]

Quick version: set keyboardActivated to true in the route's activate hook and to false in the deactivate hook.

See #52 for some more discussion.

[SergeAstapov]

I think with latest changes in the addon this is invalid approach and the one can use on-key helper like so

{{on-key '/' this.onSlash}}

Such approach is already being taught in docs on Usage page.

@lukemelia what do you think? Should we close this?