Document subsample table and presubmission hooks

[donaldcampbelljr]

We had an issue where @nleroy917 wanted to use subsamples but it was difficult to figure out how to use a subsample table AND pass the subsamples to a pipeline.

We discussed using presubmission hooks to send this data along to pipeline. This connection is not clear in the documentation.

[nleroy917]

I think this would be a great place to add that information, right in the use a subsample table section