Add contributing documentation

[sethetter]

There is a README that comes with the Slack app platform that covers a lot of good high level concepts, but having more direct and succinct documentation specific to this app would be helpful.

• Using direnv for local env vars.
• How to be able to use slack run to test local changes in our workspace.
  • Q: Does this require some special permissions? If so, document how to request those from a workspace admin.
• How changes can be deployed
  • Hopefully automated at some point, but requires workspace admin help for now.