Add a simple Perl script to update INDEX.md

Executing this .perl script (with "perl update-index.perl") will update
the INDEX.md file, fixing indentation, sort order and adding so-far
neglected recipes.

It will not commit the file, though, but leave that to the user. Who
should make sure to verify that the changes are sound.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>