Add documentation for passing data from admin form to Resource

[matthewhegarty]

Problem

It's not clear how to pass data from a custom form to the Resource. (e.g. #1553).

Solution

Updated the 'advanced usage' documentation to give an example.

Acceptance Criteria

Tested by building docs locally and checking.

[coveralls]

Coverage: 100.0%. Remained the same when pulling c50ad53 on matthewhegarty:import-doc-update into 4ac05ae on django-import-export:main.