Add debugging section

A great introduction to debugging: https://www.youtube.com/watch?v=EBbSM6DSZnA
Another good resource: https://medium.com/@diamondgfx/debugging-phoenix-with-iex-pry-5417256e1d11

Command to debug tests:
```bash
iex -S mix test --trace
```

(`mix help test` to see info on this)

Ensure your database wont timeout when debugging tests by adding an [ownership timeout](https://github.com/LDMW/app/blob/master/config/test.exs#L20) in your `config/test.exs`:
(Otherwise you will lose your database connection in 15 seconds when paused in IEx)
```
...
pool: Ecto.Adapters.SQL.Sandbox,
ownership_timeout: 10 * 60 * 1000 # 10 mins
```

Command to debug server:
```bash
iex -S mix phoenix.server
```

That's everything I think you need to know tbh