update documentation on samples.tsv

[ctb]

See commit - 3f753a1 - we could/should have a check that the number of lines in samples.tsv is matched by the number of rows in the loaded table, or something.

I figured this out by printing the sample_info in the deseq2 code, see
19ee155.

An alternative would be to save a copy of the loaded samples.tsv into the output directory.

Some documentation on what looks at & uses samples.tsv, and how, would also be a good idea :)

[bluegenes]

the validate function, which was commented out, but is back in PR #120 should handle some of the tsv formatting issues. Docs, replicate checking still needed.