Update documentation for responding with proper JSON API headers

[angelospanag]

The current response examples in the README are returning content-type in headers: text/plain; charset=UTF-8.

In order to return a proper JSON API content-type application/vnd.api+json, the following lines should be reversed:

	w.WriteHeader(http.StatusCreated)
	w.Header().Set("Content-Type", jsonapi.MediaType)

to:

	w.Header().Set("Content-Type", jsonapi.MediaType)
	w.WriteHeader(http.StatusCreated)

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[angelospanag]

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