text: Title: CHUNK_SIZE defaults to 1, so unless explicitly set, prefetching won't work
  Body: **Describe the bug**
  Not setting `CHUNK_SIZE` in your resource will cause it to default to being set to 1. Since we need to paginate the queryset to enable prefetching, this means we iterate over the queryset row by row, which prevents prefetching from working.
  
  Setting the `CHUNK_SIZE` explicitly against the resource fixes the issue and lets the prefetch work, but there's no signal that this problem could be happening to the user, which can be very confusing if you're new to prefetch and can't see why it's not working (not that I know anyone who fits that description, ahem)
  
  **To Reproduce**
  Steps to reproduce the behavior:
  1. Remove the `CHUNK_SIZE` setting from your resource
  2. Make a resource for a Django model that has a valid prefetch option, e.g. a `m2m_thing = ManyToManyField(…)`
  3. Export the resource using `MyResource().export(queryset.prefetch_related('m2m_thing')`
  4. See in the query log that one query for `m2m_thing` is run for each row of the model
  
  **Versions:**
   - Django Import Export: 2.3.0
   - Python: 3.6.9
   - Django: 3.0.8 
  
  **Expected behavior**
  * Setting the `CHUNK_SIZE` default value to something higher, e.g. 5000?
  * Possibly a warning output if `CHUNK_SIZE` isn't set but prefetches are being used on the queryset?
  * A note added to the docs against `CHUNK_SIZE`?
  