update readme v1.2

Author: soroorh

README.md

@@ -5,6 +5,7 @@ Methods for label-free mass spectrometry proteomics imputation
 **Install (R)**
 
 ```
+install.packages("devtools") # devtools is required to download and install the package
 devtools::install_github("DavisLaboratory/msImpute")
 
 ```
@@ -14,14 +15,16 @@ devtools::install_github("DavisLaboratory/msImpute")
 ```
 library(msImpute)
 
-msImpute(xna) # xna is a numeric matrix with NAs
+selectFeatures(xna)  # xna is a numeric matrix with NAs (for MAR/MNAR diagnosis only)
+xna <- scaleData(xna) 
+msImpute(xna) 
 msImpute(xna, rank.max = 2) # rank 2 approximaiton
 ```
 
-See `?msImpute` for help. 
+See user manual for help. 
 
 
 **Reference**
 
-Manuscript under preparation
+Manuscript in preparation