This can be done with a using a command like ‘search and replace’
1.Open the file in ‘vi’ editor.
2.Press Escape and type :1,$s/^/##/g
3.Press Enter
1,$s/^/##/g can be broken down as
1: To start from 1st line
$s :To end at the last line
/^ : Search for the start of the line
/##:Replace with ##
/g: Replace all occurrences