Add description of TD and ATD to readme (#38)

Co-authored-by: Jonathan de Bruin <[email protected]>

README.md

@@ -76,6 +76,17 @@ naive labeling approach (screening randomly sorted records).
 ![ASReview metrics explained](https://github.com/asreview/asreview-insights/blob/master/docs/stats_explainer.png)
 
 
+Both recall and WSS are sensitive to the position of the cutoff value and the
+distribution of the data. Moreover, the WSS makes assumptions about the
+acceptable recall level whereas this level might depend on the research
+question at hand. Therefore, [Ferdinands et al.
+(2020)](https://doi.org/10.31219/osf.io/w6qbg) proposed two new metrics: (1)
+the Time to Discover a relevant record as the fraction of records needed
+to screen to detect this record (TD); and (2) the Average Time to Discover
+(ATD) as an indicator of how many records need to be screened on average to
+find all relevant records in the dataset. 
+
+
 ## Basic usage
 
 The ASReview Insights package extends ASReview LAB with two new subcommands