Update README.md

Added R package prerequisites

UKBB_analysis/README.md

 # UK Biobank (UKBB) analysis of genetic interactions for lipids
 
-Six scripts contain R (3.6.3) code for:
+**Package prerequisites:**
+
+data.table (>=1.14.0)
+dplyr (>=1.0.5)
+MASS (>=7.3-51.5)
+
+
+**Six scripts contain R (3.6.3) code for:**
 
 1. Gene-based PTV-burden association analysis for HDLc, LDLc, total cholesterol (TC) and triglycerides (TG)
 2. Gene-based PTV-PTV burden interaction analysis for HDLc, LDLc, total cholesterol (TC) and triglycerides (TG)