Simply add it as one of the filenames for the directory index, and make sure directory indexing is turned on, as given in http://httpd.apache.org/docs/mod/mod_dir.html:

DirectoryIndex index.html index.txt /cgi-bin/index.pl
[download]

always check the docs first. Save you the time and us the effort.

-- Randal L. Schwartz, Perl hacker