Added individual-of-origin information for placenta.

.gitattributes

 *.zip filter=lfs diff=lfs merge=lfs -text
+cell_origin.csv filter=lfs diff=lfs merge=lfs -text

README.md

@@ -10,4 +10,8 @@ by [Lun _et al._ (2018)](https://doi.org/10.1101/234872).
 The `placenta_vento-tormo.zip` file contains 10X count matrices from 6 runs of placental tissue from [Vento-Tormo _et al._ (2018)](https://doi.org/10.1038/s41586-018-0698-6).
 Note that 5 placentas were obtained in the study but one sample was run on the 10X machine twice.
 Inside the zip file, each subdirectory contains a count matrix for one run prior to any CellRanger cell calling.
-These were provided by Tomas Gomes (Teichmann group).
+
+The `cell_origin.csv` file contains the individual of origin corresponding to each droplet.
+These were identified from SNPs in the transcriptome, by matching the alleles to either the fetus or mother.
+
+All files were provided by Tomas Gomes (Teichmann group).