add some more setup instructions / hints

```
app = App(
    token=os.getenv("BOT_USER_OAUTH"),
    signing_secret=os.getenv("SIGNING_SECRET"),
    name="Stoicbot"
)
CHANNEL = os.getenv("CHANNEL")
```

- Would be good to add a bit more detail in the README.md how to set up Slack.
- Add a requirements-dev.txt to include pytest
- Add an .env-template file with the env variables to be set