explain platform_unit to data submitters

[rosibaj]

submitting_molecular data.m
specificlaly about the platform_unit

to be precise: only for non-multiplex sequencing (ie, without using sample barcode) platform unit has one-to-one relationship with read group. In essense, a read group is equivalent to a sequencing lane of a flowcell.

In the past, we saw a variety of ways how read group and platform unit were populated likely due to lack of clarity of either the definition or documentation. There are definitely some confusions around read group, platform unit and their relationship, here is one such discussion: https://gatkforums.broadinstitute.org/gatk/discussion/12292/understanding-read-groups, in which the user suggested Broad to make a video to explain the terminologies. I think a good diagram would be definitely helpful.

Originally posted by @junjun-zhang in #254