add a tip about configuring apache to run ikiwiki.cgi

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+Many ikiwiki examples name the [[cgi]] "ikiwiki.cgi", and put it somewhere
+like `~/public_html/ikiwiki.cgi`, or `/var/www/wiki/ikiwiki.cgi`.
+
+If you follow those examples, you may find that when trying to edit a page
+in your wiki, you see the raw contents of the ikiwiki.cgi program. Or get a
+permission denied problem.
+
+This is because apache is generally not configured to run cgi scripts
+unless they're in `/usr/lib/cgi-bin/`. While you can put ikiwiki.cgi in
+there if you like, here's how to configure apache (version 2) to run `.cgi`
+programs from anywhere.
+
+These instructions are for Debian systems, but the basic apache
+configuration should work anywhere.
+
+* Edit /etc/apache2/apache2.conf and add a line like this:
+
+       AddHandler cgi-script .cgi
+
+* Find the "Options" line for the directory where you've put the
+  ikiwiki.cgi, and add "ExecCGI" to the list of options. For example, if
+  ikiwiki.cgi is in /var/www/, edit `/etc/apache2/sites-enabled/000-default`
+  and add it to the "Options" line in the "Directory /var/www/" stanza.
+  Or, if you've put it in a `~/public_html`, edit
+  `/etc/apache2/mods-available/userdir.conf`.