Clarify Stylesheet readme: Multiple markers in an OccursUnder

[aunger]

Please update the stylesheet README.md file. It isn't clear about the meaning of an OccursUnder property listing multiple markers. I think it must mean that if (and only if) one of the OccursUnder markers appears in the USFM, that at least one of that type of marker must occur before the marker being described. However, on first read, this is unclear, and I'm still not sure I have it right.