Implement /help command

[billyao021031]

Description

I created a helpCommand.ts in the .commands folder and link it to the index.ts for app to listen to the command. The helpCommandHandler function will wait for a '/help' command and then return with an ephemeral message that contains a link to the README.md in our current working repository.

This PR closes #25.

Developer Testing

Testing done:

• Manually tested the '/help' command in the our development slack bot.
• Implemented unit tests for the new feature

Reviewer Testing

Steps to view/test Feature:

1. In the Dev slack, command '/help' in any channel that minerva is in. If minerva is not in a channel, you can add them by pinging them (sending @minervadev in a message)
2. Check if Minvera responds to your command, and make sure the link to README works.

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This change is 

[billyao021031]

! Please deploy this to development and check if the command works as expected before you squash and merge. I'm not 100% sure if the way the link is formatted is the right way to do so, so it would be good to verify that.

Reviewed 1 of 1 files at r2, all commit messages. Reviewable status: :shipit: complete! all files reviewed, all discussions resolved (waiting on @jeffrey-huang-yz, @Panizghi, @PCModeActivate, @shirleyfyx, @slightlyskepticalpotat, @wxp02, and @yxyyeah)

I tested it out before re-request review, the link was working the way it should be