Update README.md

README.md

@@ -81,6 +81,7 @@ To build a Debian package of this project, follow these steps:
 To change the atime of the files belonging to a package 
 you can run something like this:
 ```
+sudo apt -y install cowsay
 export DATE=$(date -d "7 days ago" +%Y%m%d%H%M)
 for file in `dpkg -L cowsay` ; do sudo touch -a -t $DATE $file; done
 ```