Add note on buttons

[davidkarlsen]

For enabling buttons you need a verification token.
This is only possible if you create hubot as a custom app and don't select the "custom integration" option which already exists for hubot and appears if you search for hubot in the apps section.

This issue: slackapi/hubot-slack#316 is also relevant.

One thing to note is that button actions require the bot to be available on the internet, while without this option it can work behind a firewall with a http-proxy.