You can simple add a column at the front of your matrix and use dlmcell to write in a text file.
Suppose A is you cell matrix which contains the data which is in your .txt file.
A={'a','b','c','d','e';'f','g','h','i','j';'k','l','m','n','o';'p','q','r','s','t'}
A =
4×5 cell array
'a' 'b' 'c' 'd' 'e'
'f' 'g' 'h' 'i' 'j'
'k' 'l' 'm' 'n' 'o'
'p' 'q' 'r' 's' 't'
A=[arrayfun(@(x) num2str(x) ,[-1,1:size(A,1)-1] ,'uni',0)' A]
A =
4×6 cell array
'-1' 'a' 'b' 'c' 'd' 'e'
'1' 'f' 'g' 'h' 'i' 'j'
'2' 'k' 'l' 'm' 'n' 'o'
'3' 'p' 'q' 'r' 's' 't'
