added description for data versioning

Author: Jingjing Tang

nchs_mortality/DETAILS.md

@@ -53,3 +53,9 @@ refers to an epiweek). However, NCHS reports their weekly data from Saturday to
 Saturday. We assume there is a one day shift. For example, they report a death counts 
 for Alaska in a week starting from date D, we will report the timestamp of this report 
 as the corresponding epiweek of date(D + 1).
+
+### Data Versioning
+We check for updates for NCHS mortality data every weekday as how it is reported by 
+CDC and stash them on AWS. We merge them with the data stashed previously and keep track of 
+the changes. Every Mondays, we upload both the most recent version of data and the 
+changes made over the past week due to backfill to our API.