Add cheatsheet for cp backup file with date

cheat/cheatsheets/cp

@@ -6,3 +6,6 @@ cp -r ~/Desktop/cruise_pics/ ~/Pictures/
 
 # Create a copy but ask to overwrite if the destination file already exists
 cp -i ~/Desktop/foo.txt ~/Documents/foo.txt
+
+# Create a backup file with date
+cp foo.txt{,."$(date +%Y%m%d-%H%M%S)"}