Add end-to-end slack adapter example

[timkpaine]

decoupling from https://github.com/Point72/csp/pull/22 so as to not mess with https://github.com/Point72/csp/pull/26

• [ ] show full end-to-end setup
• [ ] document what oauth scopes are necessary for what functionality
• [ ] define minimum subset and make sure supported in code (e.g. things should turn off without erroring if missing scopes)
• [ ] make a minimal chatbot