Contribution guide #21

I think a contribution section should be added to the readme to walk new users through things like:

• adding a new command
• adding a new scheduled job
• calling slack api methods and highlight interesting ones
• highlight interesting slack events they might want to respond to
• how to get user or channel IDs
• how to reply privately, publicly or initiate a conversation
• etc.

We want it to be as easy as possible to go from idea -> code -> pull request to encourage more contributors to get involved.