Improve documentation regarding response.headers

[jonyyz]

The Redaxios documentation refers us to the Axios documentation but that's not correct for some features. In the case of response.headers if we were to follow the Axios documentation to retrieve say Content-Disposition it would look like this:

response.headers['Content-Disposition']

However, redaxios has forwarded the fetch api response interface that is like so:

response.headers.get('Content-Disposition')

It would be nice to have better documentation that points out the differences between Axios and Redaxios and further highlights that redaxios is definitely not a drop-in replacement for axios.