message: add link to sample code
diff: # 1.0.5
  -* Add support for using the new [Block Kit modals](https://api.slack.com/block-kit/surfaces/modals)
  +* Add support for using the new [Block Kit modals](https://api.slack.com/block-kit/surfaces/modals). Check out [this sample code for examples of using them](https://github.com/howdyai/botkit/blob/master/packages/testbot/features/slack_modals.js).
  * Update to the latest version of the [@slack/web-api](https://www.npmjs.com/package/@slack/web-api), granting access to recently added methods like views.open, views.update
  * Fix: previous versions incorrectly labeled incoming `block_actions` and `interactive_message` events as `message` events. This version corrects this - `controller.on('block_actions',...)` and `controller.hears(pattern,'block_actions',...)` should now work.
  * Expand ability to "hear" block actions and use them in conversations. Selections from dropdowns and datepickers can now be heard by the bot and will be included as `message.text` in incoming messages. Thanks to [@sfny](https://github.com/sfny) for the [pull request](https://github.com/howdyai/botkit/pull/1809)
  