Update README to avoid error on route in Sinatra
Update README to avoid error on route in Sinatra

Since Goatmail is only available in development, if you mount the route
`/inbox` in other environment it will generate an error

```
config.ru:in `block (2 levels) in <main>':
  uninitialized constant Goatmail (NameError)
```

This fixes the instructions on how to mount the route properly.

tyabe/goatmail,tyabe/goatmail,tyabe/goatmail