Update README.md

It took me a while to make the connection between 

```ruby
payload[:host] = request.host
```

in the controller and

```ruby
event.payload[:host]
```
in the custom_options lambda. This or something like it should make it a little more clear.
Could you format the event.payload[] bit in backticks so it shows up as inline code please? It makes the docs easier to scan quickly `event.payload[]`