Bug 1183571 - tell people they need to enable the CGI module. r=LpSolit

docs/en/rst/installing/apache.rst

@@ -77,6 +77,14 @@ or, if not found, :file:`index.html` if someone only types the directory name
 into the browser; and allows Bugzilla's :file:`.htaccess` files to override
 some global permissions.
 
+On some Linux distributions you will need to enable the Apache CGI
+module. On Debian/Ubuntu, this is done with:
+
+:command:`sudo a2enmod cgi`
+
+If you find that the webserver is returning the Perl code as text rather
+than executing it, then this is the problem.
+
 .. _apache-mod_perl:
 
 Apache with mod_perl