Update README.md

README.md

 # How to install
-start R
+Start R
 
-load the library called devtools
-'''library(devtools)'''
-install WFRTools:
-'''install_git("https://git.wur.nl/frans004/WFRTools.git")'''
+Load the library called devtools
+```R
+library(devtools)
+```
+Install WFRTools:
+```R
+install_git("https://git.wur.nl/frans004/WFRTools.git")
+```