Describe how to extend mime.types in types docs

[anatoly techtonik]

The correct way to extend mime.types with types is to include it in the same section:

include mime.types;
types {
    # here is additional types
}

From here: ​http://stackoverflow.com/questions/16789494/extending-default-nginx-mime-types-file

But this is not documented in: ​http://nginx.org/en/docs/http/ngx_http_core_module.html#types