Add example of additional_codecs setting

[jaalcorn]

Add an example using the additional_codecs setting to clarify what the document is stating about using two codecs and reinforce the codec setting's tie back to the Content-Type setting

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/

[andsel]

Hi @jaalcorn thank's for contributing.
I think the es_bulk codec is quite particular, may be worth to see the configuration with a main codec and another more specific, so let understand that the additional_codec are used when no content-type codec is matched, for example:

http {
  codec => json
  additional_codecs => {"application/text", "plain"}
}