add `help` command

[jorydotcom]

It's not obvious how to use the bot, esp. on first install. Should add a help command that says what the bot does and how to use it.

[jorydotcom]

Also perhaps typing /thanksbot should allow someone to record a message without opening a dm

[mjchamplin]

In general I prefer this pattern, especially once I learn how to use a bot.

[jorydotcom]

I think so too. it's so simple anyway, opening a dm almost feels like too much friction / an extra step.