Improve documentation regarding response.headers

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:

```js
response.headers['Content-Disposition']
```

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

```js
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.