Indexing Unix command

[Rodolfo Medina <rodolfo.medina@gmail.com>]

Dear Unix-experts,

with the following simple script:

#!/bin/sh

cat index.idx | sort > index.ind

I sort the contents of a file and write it in another file.  Now, I want that a
small vertical space, i.e. an empty line or two, were inserted before all the
words that start with a new letter.  It's clear that I'm working at some
indexing mechanism.  Please suggest whoever can what code should I add to that
one above in order to do what I want.

Thanks in advance for any help,

Rodolfo