Add fossil CGI documentation

docs/fossil-cgi.txt

@@ -0,0 +1,21 @@
+Fossil as a CGI server
+======================
+
+Create the following CGI file in /usr/lib/cgi-bin/fossil:
+
+   #!/usr/local/bin/fossil
+   directory: /srv/fossil
+   repolist
+   setenv: SCRIPT_NAME
+
+This should be executable, and CGI should be enabled in Apache with the
+`a2enmod cgi` command.
+
+Edit the Apache configuration to add the following lines:
+
+   RewriteEngine on
+   RewriteCond %{REQUEST_URI} "!^/cgi-bin/.*$"
+   RewriteCond %{REQUEST_URI} "!^/robots\.txt$"
+   RewriteRule "^/(.*)$" "/cgi-bin/fossil/$1" [NC,PT]
+
+This will allow the fossil site to be served from the site root.