add documentation of how to best split files in preparations

[antgonza]

A user pointed out that it will be great to have some documentation of how to best split prep info files. This can be super helpful in specific for per_sample_FASTQ as they can be ordered any way the user wants.

cc: @johannesbjork

[antgonza]

It will be good to add some of this information to the documentation:

If we imagine the worst case scenario, that we have in all our columns the default 'not applicable' NULL value (https://www.ebi.ac.uk/ena/about/missing-values-reporting), and that XML adds 4 times the size due to its formatting (how EBI-ENA expects the submission); then we will have len('not applicable') * 50 * 4 = 2800 (2.8K) per sample; then we can divide that by the 10M (max submission for EBI-ENA) so a preparation can not have than 3.5K samples. Note that the more metadata columns, the less samples you can submit in a single go.