Add citation for leaf area data to supplemental

[Aariq]

@embruna will put the relevant data on figshare or something to get a doi, then add citation to supplemental.Rmd

[embruna]

almost there [repo]. The data are all there, but I have 1-2 minor questions (e.g., about 1 column name related to seed set, 1 plant missing measurements in one of the 2 time periods) to deal with by looking at the original data sheets. It will be on figshare friday with a doi