Add information regarding Authentication to the readme.

[brokenprogrammer]

When authenticating towards the fortnox api you first get an authorization code which you then have to turn into an access token before you can use the api.

We have support for this within the SDK but it is not yet documented within our readme.

The service: AuthorizationService.cs have a method called GetAccessTokenAsync which is pretty self explanatory.
Example usage:

var accessToken = (await AuthorizationService.GetAccessTokenAsync(authorizationCode, clientSecret)).Authorization.AccessToken;