[Doc] Markdown support

[bentcoder]

Hi,

Could you please add an example where a message contains markdown? I tried this but I think the API has changed a bit.

Thanks

[shigedangao]

Were you able to make it work ?

[rockwotj]

I think you need to use markdown_text instead of text in the chat API: https://api.slack.com/methods/chat.postMessage#arg_markdown_text