Add documentation for customizing retrieval process

[jvence]

Firstly, thank you for creating one of the most useful project I have seen for Q&A and NLP. It would be great to add documentation for configuring the retrieval with:
1- Our own data imported into ElasticSearch
2- Custom retrievals.

[victordibia]

Hi @jvence

Thanks for the issue. Thats a good idea, Im tracking it and will post updates as I make progress here.