Remove default generated route

From the README:

    A route is needed for the endpoint which receives `POST` messages. Currently,
    the route is automatically appended to the route table like so:

    ```ruby
    email_processor POST /email_processor(.:format)   griddler/emails#create
    ```

    **NOTE: This behavior is deprecated and will be removed by version 0.7.0 in favor
    of manually adding the route.**

    To manually add the route, in `config/routes.rb` you may either use the provided
    routing method `mount_griddler` or set the route explicitly. Examples:

    ```ruby
    # mount using default path
    mount_griddler

    # mount using a custom path
    mount_griddler('/email/incoming')

    # the DIY approach:
    post '/email_processor' => 'griddler/emails#create'
    ```



We want to remove the generated route and update the README to know that.