Add note about :content-type encoding

[danielcompton]

I was confused about this and assumed I could also use :body. I thought this might help others too. LMK if I've misunderstood something here.

[dakrone]

:content-type is for specifying the content type header for:body, for example, {:content-type :json} means that the Content-Type: application/json header will be added to the request.