Specify in docs that vcf file needs accompanying index file

[zrcjessica]

Include in docs explicit instructions to have both a .vcf.gz and .vcf.gz.tbi file

[aryarm]

just attaching a note to this that it's kinda already in the config file here but not in the docs here

ideally, we would have a pre-processing validation check that errors out if the index isn't there to begin with